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Title: Anatomy of the Cat

Author: Jacob Reighard
        H. S. Jennings

Illustrator: Louise Burridge Jennings

Release Date: December 1, 2018 [EBook #58394]

Language: English

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ANATOMY OF THE CAT

BY

JACOB REIGHARD
Professor of Zoology in the University of Michigan

AND

H. S. JENNINGS
Instructor in Zoology in the University of Michigan

WITH
ONE HUNDRED AND SEVENTY-THREE ORIGINAL FIGURES
DRAWN BY
LOUISE BURRIDGE JENNINGS

NEW YORK
HENRY HOLT AND COMPANY
1901

Copyright, 1901,
BY
HENRY HOLT & CO.

ROBERT DRUMMOND, PRINTER, NEW YORK.

[iii]

PREFACE.

Although the cat has long been in common use for the practical study of mammalian anatomy, a clear, correct, not too voluminous account of its structure, such as should be in the hands of students in the laboratory, has remained a desideratum. A number of works have been published on the cat, some of them of much value, yet there is none which fulfils exactly the conditions mentioned. The books which have appeared on this subject are the following:

1. Strauss-Durckheim, H. Anatomie descriptive et comparative du Chat. 2 vols. Paris, 1845.

2. Mivart, St. George. The Cat: an Introduction to the Study of Back-boned Animals, especially Mammals. New York, 1881.

3. Wilder, Burt G., and Gage, Simon H. Anatomical Technology as applied to the Domestic Cat. New York, 1882.

4. Gorham, F. P., and Tower, R. W. A Laboratory Guide for the Dissection of the Cat. New York, 1895.

5. Jayne, H. Mammalian Anatomy. Vol. I. Philadelphia, 1898.

The first of these works treats only of the muscles and bones, and is not available for American students. Its excellent plates (or Williams’s outline reproductions of the same) should be in every laboratory.

The second book named is written in such general terms that its descriptions are not readily applicable to the actual structures found in the dissection of the cat, and experience has shown that it is not fitted for a laboratory handbook. It contains, in addition to a general account of the anatomy of the cat, also a discussion of its embryology, psychology, palæontology, and classification.

[iv]

The book by Wilder and Gage professedly uses the cat as a means of illustrating technical methods and a special system of nomenclature. While of much value in many ways, it does not undertake to give a complete account of the anatomy of the animal.

The fourth work is a brief laboratory guide.

The elaborate treatise by Jayne, now in course of publication, is a monumental work, which will be invaluable for reference, but is too voluminous to place in the hands of students. At present only the volume on the bones has been published.

As appears from the above brief characterization, none of these books gives a complete description of the anatomy of the cat in moderate volume and without extraneous matter. This is what the present work aims to do.

In the year 1891-92, Professor Reighard prepared a partial account of the anatomy of the cat, which has since been in use, in typewritten form, in University of Michigan classes. It has been used also at the Universities of Illinois, Nebraska, and West Virginia, and in Dartmouth College, and has proven so useful for college work in Mammalian Anatomy that it was decided to complete it and prepare it for publication. This has been done by Dr. Jennings.

The figures, which are throughout original, are direct reproductions of ink drawings, made under the direction of Dr. Jennings by Mrs. Jennings.

The book is limited to a description of the normal anatomy of the cat. The direct linear action of each muscle taken alone has been given in the description of muscles; other matters belonging to the realm of physiology, as well as all histological matter, have been excluded. It was felt that the monumental work of Jayne on the anatomy of the cat, now in course of publication, forms the best repository for a description of variations and abnormalities, so that these have been mentioned in the present volume only when they are so frequent as to be of much practical importance.

Except where the contrary is stated, the descriptions are based throughout on our own dissections and observations and are in no sense a compilation. For this reason we have not[v] thought it necessary to collect the scattered references to the anatomy of the cat that may occur in the literature. A collection of such references may be found in Wilder and Gage’s Anatomical Technology. In addition to the works already referred to, we have of course made use of the standard works on human and veterinary anatomy. Among these should be mentioned as especially useful the Anatomie des Hundes by Ellenberger and Baum. Other publications which have been of service in the preparation of the work are Windle and Parson’s paper On the Myology of the Terrestrial Carnivora, in the Proceedings of the Zoological Society of London for 1897 and 1898, T. B. Stowell’s papers on the nervous system of the cat in the Proceedings of the American Philosophical Society (1881, 1886, 1888) and in the Journal of Comparative Neurology (vol. I.), and F. Clasen’s Die Muskeln und Nerven des proximalen Abschnitts der vorderen Extremität der Katze, in Nova Acta der Ksl. Leop.-Carol. Deutschen Akademie der Naturforscher, Bd. 64.

Nomenclature.—The question of nomenclature has been one of difficulty. What is desired is a uniform set of anatomical names,—a system that shall be generally used by anatomists. At present the greatest diversity prevails as to the names to be applied to the different structures of the body. The only set of terms which at the present time seems to have any chance of general acceptance is that proposed by the German Anatomical Society at their meeting in Basel in 1895, and generally designated by the abbreviation BNA. This system has therefore been adopted, in its main features, for use in the present work. It seems impossible at the present time, however, to impose any one set of terms absolutely upon anatomists of all nations, and we have felt it necessary to use for certain familiar structures, in place of the BNA terms, names that have come to have a fixed place in English anatomy, and may almost be considered component parts of the English language. The German anatomists have expressly recognized the fact that this would be to a greater or less degree necessary among anatomists of different nations, and have characterized their list as for the present tentative, and[vi] capable of farther development. The only purpose of a name is that it shall furnish a key to a common understanding; where the BNA name does not furnish such a key to English readers, and where there is a term in established English usage that does serve this purpose and seems unlikely to be supplanted, we have used the latter. But we have endeavored to make the number of these exceptions as small as possible, and in such cases we have usually cited at the same time the term proposed by the German society, followed by the abbreviation BNA. When, on the other hand, we have adopted a BNA term for which there is also a commonly used English equivalent, the latter has likewise usually been cited in parenthesis.

In deciding whether or not to use in a given case the BNA term many difficult cases arose. Will the common English name innominate bone (os innominatum) be replaced by the BNA term os coxæ or coxal bone? We have held this to be highly improbable, and have therefore used the term innominate bone, merely citing os coxæ (BNA) as a synonym. In the same way we have used centrum as a designation of a part of a vertebra, in place of corpus (BNA); premaxillary bone or premaxilla in place of os incisivum (BNA); malar bone in place of os zygomaticum (BNA); trapezoid as a name of one of the bones of the carpus, in place of os multangulum minus (BNA), etc. In other cases where it has seemed probable that the BNA term would come into common use, though now unfamiliar, this and the more common English expression are both used or used alternatively; such has been the case, for example, with the Gasserian ganglion or semilunar ganglion (BNA). In naming the cerebral sulci and gyri the system in use for man is not well fitted for bringing out the plan of those in the brain of the cat, so that it was necessary to reject the BNA names for these structures.

As to the use of the Latin terms and their equivalents in English form, we have made a practice of employing in the text sometimes one, sometimes the other; this has the advantage of giving variety, and of impressing the interchangeability of the Latin and English forms on the mind of the student. Where a given structure is called by two equally well-known[vii] names, we have used both, holding that the student should become familiar with each and recognize their identity of meaning.

In general we have maintained the principle that the primary purpose of such a work as the present is not to illustrate or defend any particular system of nomenclature, but to aid in obtaining a knowledge of the structures themselves. With this end in view, we have used such terms as would in our judgment best subserve this purpose, making the BNA system, as the one most likely to prevail, our basis. In applying the system we have had to keep in mind a number of sometimes conflicting principles. In some cases the judgment of other anatomists will doubtless differ from our own; but this we feel to be inevitable. The matter of an absolutely uniform nomenclature is not ripe for settlement at the present time.

Some further explanation is needed in regard to the topographical terms, or terms of direction, used in the present work. We have adopted the BNA terms in this matter also. The terms superior, inferior, anterior, and posterior have been avoided, as these terms do not convey the same meaning in the case of the cat as they do in man, owing to the difference in the posture of the body. In place of these terms are used dorsal and ventral, cranial and caudal. As terms of direction these, of course, must have an absolutely fixed meaning, signifying always the same direction without necessary reference to any given structure. For example, cranial means not merely toward the cranium, but refers to the direction which is indicated by movement along a line from the middle of the body, toward the cranium; after the head or cranium is reached, the term still continues in force for structures even beyond the cranium. Thus the tip of the nose is considered to be craniad of the cranium itself. Lateral signifies away from the middle plane; medial toward it. Inner and outer or internal and external are used only with reference to the structure of separate organs, not with reference to the median plane of the body.

In describing the limbs the convexity of the joint (the elbow or knee) is considered as dorsal, the concavity being therefore[viii] ventral. Medial refers to that side of the limb which in the normal position is toward the middle of the body; lateral to the outer side. Terms of direction which are derived only from the structure of the limb itself are in some cases more convenient than the usual ones. In the fore limbs the terms radial (referring to the side on which the radius lies) and ulnar (referring to the side on which the ulna lies) are used; in the hind limbs the terms tibial and fibular are used in a similar manner. Distal means toward the free end of a limb or other projecting structure; proximal, toward the attached end.

For all these terms an adverbial form ending in -ad has been employed. Experience has shown this to be very useful in practice, and while not expressly recommended by the BNA, it is not condemned. Terms ending in -al are therefore adjectives; those ending in -ad are adverbs.

In compounding these terms of direction, the hyphen has been omitted in accordance with the usage recommended by the Standard Dictionary. Thus dorsoventral is written in place of dorso-ventral, etc. The student will perhaps be assisted in understanding these compounds if he notes that the first component always ends in -o, so that the letter o practically serves the purpose of a hyphen in determining how the word is to be divided.

In one particular the BNA nomenclature is not entirely consistent. While recommending or at least permitting the use of the general terms dorsal and ventral in place of the human posterior and anterior, and cranial and caudal in place of superior and inferior, it retains the words anterior, posterior, superior, and inferior as parts of the names of definite organs. For example, we have the muscle serratus anterior in place of serratus ventralis; serratus posterior inferior in place of serratus dorsalis caudalis. This is very unfortunate, from a comparative standpoint, but we have felt it necessary to retain the BNA terms in order that the structures of the cat may receive the same names as the corresponding structures of man.

In the matter of orthography we have endeavored to follow the best English anatomical usage, as exemplified in Gra[ix]y’s Human Anatomy,—therefore writing peroneus in place of peronæus, pyriformis in place of piriformis, etc.

The book is designed for use in the laboratory, to accompany the dissection and study of the structures themselves. Anatomy cannot be learned from a book alone, and no one should attempt to use the present work without at the same time carefully dissecting the cat. On the other hand, anatomy can scarcely be learned without descriptions and figures of the structures laid bare in dissection, so that this or some similar work should be in the hands of any one attempting to gain a knowledge of anatomy through the dissection of the cat.

The figures have all been drawn from actual dissections, and have been carefully selected with a view to furnishing the most direct assistance to the dissector. It is hoped that no figures are lacking that are required for giving the students the necessary points of departure for an intelligent dissection of any part of the body. The fore limb is illustrated somewhat more fully than the hind limb, because it was thought that the fore limb would usually be dissected first; the hind limb will be easily dissected, with the aid of the figures given, after the experience gained in dissecting the fore limb.

As the book is designed to accompany the dissection of the specimen in the laboratory, it was deemed best to give succinct specific directions for the dissection of the different systems of organs, together with suggestions as to methods of preserving and handling the material. These are included in an appendix.

[x]
[xi]

CONTENTS.

[xvi]
[xvii]

LIST OF ILLUSTRATIONS.

[1]

ANATOMY OF THE CAT.

THE SKELETON OF THE CAT.

The skeleton of the cat consists of 230 to 247 bones exclusive of the sesamoid bones (44) and the chevron bones (8). These are divided as follows: head 35-40, vertebral column 52-53, ribs 26, sternum 1-8, pelvis 2-8, upper extremities 62, lower extremities 54-56. The number of bones varies with the age of the individual, being fewer in the old than in the young animal, owing to the fact that in an old animal some bones that were originally separate have united.

I. THE VERTEBRAL COLUMN. COLUMNA VERTEBRALIS.

The vertebral column, spinal column, or back-bone, consists of a varying number of separate bones, the vertebræ. At its cranial end are seven vertebræ (cervical, Fig. 1, c) which are without ribs and support the head; caudad of these are thirteen rib-bearing vertebræ (thoracic, Fig. 1, m); caudad of these are seven that are again without ribs (lumbar, Fig. 1, o); these are followed by three vertebræ (sacral, Fig. 1, x) which are united into a single bone, the sacrum, which supports the pelvic arch. Following the sacral vertebræ are twenty-two or twenty-three small ribless vertebræ which support the tail (caudal, Fig. 1, y).

Fig. 1.—Skeleton of Cat.

a, skull; b, hyoid; c, cervical vertebræ; d, clavicle; e, scapula; f, sternum; g, humerus; h, radius; i, ulna; j, carpus; k, metacarpus; l, phalanges; m, thoracic vertebræ; n, ribs; o, lumbar vertebræ; p, innominate bones; q, femur; r, patella; s, fibula; t, tibia; u, tarsus; v, metatarsus; w, phalanges; x, sacrum; y, caudal vertebræ.

Thoracic Vertebræ. Vertebræ thoracales

(Fig. 4).—The thoracic vertebræ are most typical, and the fourth one of these may therefore be first described (Figs. 2 and 3). It forms an oval ring which has numerous processes and surrounds an opening which is the vertebral foramen (a). The ventral one-third[2]
[3] of this ring is much thickened and forms the centrum or body (corpus) (b) of the vertebra. The centrum is a semicylinder, the plane face of which bounds the vertebral canal, while the curved surface is concave longitudinally and is directed ventrad. The dorsal plane surface of the centrum is marked by a median longitudinal ridge on either side of which is an opening (nutrient foramen) for a blood-vessel. The ends are nearly plane, the caudal being slightly concave; they are harder and smoother than the other surfaces. They may be easily separated in a young specimen as thin plates of bone known as epiphyses.

Fig. 2.—Fourth Thoracic Vertebra, Cranial End.

Fig. 3.—Fourth Thoracic Vertebra, Side View.

a, vertebral foramen; b, centrum; c, caudal, and d, cranial, costal demifacets; e, radix or pedicle; f, lamina; g, transverse process; h, cranial articular facet; i, caudal articular facet; j, caudal articular process; k, spinous process.

At the caudal end of the centrum, at its dorsolateral angle, is a smooth area on each side continuous with the surface of the epiphysis and bounded dorsolaterally by a sharp ridge of bone (c). It is a costal demifacet. In corresponding positions at the cranial end of the centrum are two demifacets not limited by bony ridges (d). When the centra of two contiguous thoracic vertebræ are placed together in the natural position the cranial costal demifacets of one together with the caudal demifacets of the other form two costal facets (Fig. 4, e), one on each side, and each receives the head of a rib.

The dorsal two-thirds of the vertebral ring forms the vertebral arch which is continued dorsally into the long, bluntly pointed spinous process (Figs. 2 and 3, k) for attachment of muscles.

[4]

The vertebral arch (each half of which is sometimes called a neurapophysis) rises on each side from the cranial two-thirds of the dorsolateral angle of the centrum, as a thickened portion, the radix or pedicle (Figs. 2 and 3, e), which forms the ventral half of the lateral boundary of the vertebral canal. From the dorsal end of each radix a flat plate of bone, the lamina (f), extends caudomediad to join its fellow of the opposite side and form the vertebral arch. Owing to the fact that the radix rises from only the cranial two-thirds of the centrum there is left in the caudal border of the vertebral arch a notch bounded by the radix, the lamina, and the centrum. There is also a slight excavation of the cranial border of the radix. When the vertebræ are articulated in the natural position, these notches form the intervertebral foramina (Fig. 4, d), for the exit of the spinal nerves.

At the junction of radix and lamina the arch is produced craniolaterad into a short process, the transverse process (g), knobbed at the end. On the ventral face of its free end the transverse process bears a smooth facet, the transverse costal facet or tubercular facet (Fig. 4, c), for articulation with the tubercle of a rib.

On the dorsal face of each lamina at its cranial border is a smooth oval area, the cranial articular facet (superior articular facet of human anatomy) (Figs. 2 and 3, h). Its long axis is oblique and it looks dorsolaterad. The slight projections of the cranial edge of the laminæ on which the facets are situated are the inconspicuous cranial articular processes (prezygapophyses).

On the ventral surface of each lamina at the caudal border, near the middle line is a similar area, the caudal articular facet (inferior articular facet of human anatomy) (i); these occupy the ventral surfaces of two projections which form the caudal (inferior) articular processes (postzygapophyses) (j). These are separated by a median notch. When the vertebræ are in their natural position the caudal articular facets lie dorsad of the cranial facets and fit against them. They thus strengthen the joint between contiguous vertebræ, while permitting slight rotary motion.

[5]

Fig. 4.—Thoracic Vertebræ, Side View.

a, spinous processes; b, cranial articular processes; c, transverse costal facets; d, intervertebral foramina; e, costal facets; f, accessory processes; g, mammillary processes; h, caudal articular processes.

Differential Characters of the Thoracic Vertebræ (Fig. 4).—Following the thoracic vertebræ caudad there is to be seen a gradual increase in the size of the centra brought about by an increase in their craniocaudal and transverse measurements. The dorsoventral measurements remain nearly the same. The costal facets (Fig. 4, e) shift caudad so that on the eleventh, twelfth, and thirteenth thoracic vertebræ each lies entirely on the cranial end of its centrum, while the caudal end[6] of the centrum immediately preceding is not marked by any part of it. In the eleventh thoracic vertebra each costal facet is usually still confluent with the smooth cranial end of the centrum. In the twelfth vertebra the facets are separated by smooth ridges from the cranial end of the vertebra, while in the thirteenth vertebra they are separated by rough ridges.

The spinous processes (a) of the first four are of about the same length. They then decrease in length to the twelfth, while the twelfth and thirteenth are slightly longer than the eleventh. The first ten slope more or less caudad, while the spinous process of the tenth (anticlinal) vertebra is vertical and those of the eleventh, twelfth, and thirteenth point craniad.

Each of the transverse processes of the seventh thoracic vertebra shows a tendency to divide into three tubercles; one of these is directed craniad, the mammillary process (or metapophysis), one caudad, the accessory process (or anapophysis), while the third (transverse process proper) looks ventrad and bears the transverse costal facet. This division becomes more prominent in the succeeding vertebræ, being most marked in the ninth and tenth. In the eleventh, twelfth, and thirteenth vertebræ the mammillary (g) and accessory (f) processes are very pronounced, while the transverse costal facet and that part of the transverse process which bears it have disappeared. The ribs of the eleventh, twelfth, and thirteenth vertebræ are thus attached to their respective centra by their heads alone.

The cranial articular processes (b) are prominent on the first two thoracic vertebræ; back of these they are very small as far as the eleventh, so that the articular facets seem to be borne merely upon the dorsal surface of the cranial edge of the laminæ. In the eleventh, twelfth, and thirteenth the cranial articular processes are large, bearing the articular facets on their medial surfaces, while the mammillary processes appear as tubercles on the lateral surfaces of the articular processes. The caudal articular processes (h) are prominent in the first thoracic, then smaller until the tenth is reached; in the tenth, eleventh, twelfth, and thirteenth they are large and their facets are borne laterally, so as to face the corresponding cranial[7] facets. Thus from the tenth to the thirteenth thoracic vertebra rotary motion is very limited, owing to the interlocking of the articular processes.

The Lumbar Vertebræ. Vertebræ lumbales

(Fig. 5).—The last thoracic vertebræ form the transition to the typical lumbar vertebræ. These are larger than the thoracic vertebræ. The centra are of the form of the centra of the thoracic vertebræ, and increase in length to the sixth, but the seventh is about the length of the first. They increase in breadth to the last.

Fig. 5.—Lumbar Vertebræ.

a, cranial articular processes; b, mammillary processes; c, caudal articular processes; d, accessory processes; e, transverse processes; f, spinous processes.

[8]

The cranial articular processes (Fig. 5, a) are prominent and directed craniodorsad; they have the facets on their medial surfaces, while their dorsolateral surfaces bear the mammillary processes (b) as prominent tubercles. The caudal articular processes (c) are likewise large; their facets look laterad. When the vertebræ are articulated they are received between the medially directed cranial processes.

The accessory processes (d) are well developed on the first vertebra, diminish in size to the fifth or sixth, and are absent on the seventh and sometimes on the sixth.

The transverse processes (more properly pseudo-transverse processes) (e) arise from the lateral surface of the centra; are flat and are directed ventrocraniolaterad. The first is small, and they increase in length and breadth from the first to the sixth, those of the last being slightly smaller than in the sixth. The free ends of the last four are curved craniad.

The spinous processes (f) are flat and directed craniodorsad. They increase in length to the fifth and then decrease. The first five are knobbed at the end. In a dorsal view the spinous process and cranial articular processes of each vertebra are seen to interlock with the caudal articular processes and accessory processes of the preceding vertebra in such a way as to prevent rotary motion, and this arrangement may be traced craniad as far as the eleventh thoracic vertebra.

Sacral Vertebræ. Vertebræ sacrales

(Figs. 6 and 7).—The three sacral vertebræ are united in the adult into a single bone, the os sacrum, or sacrum. In a kitten the three vertebræ are separate, while in an animal almost mature the first two are united and the third is still separate. The sacrum lies between the last lumbar and the first caudal vertebræ and articulates laterally with the two innominate bones. It is pyramidal, with the base of the pyramid directed craniad, and is perforated by a depressed longitudinal canal, the sacral canal, which is a continuation of the vertebral canal, and by four large foramina dorsally and four ventrally. It may be described as having a cranial end or base and a caudal end or apex, a dorsal, a ventral, and two lateral surfaces.

The base is slightly oblique and presents a smooth transversely[9] oval articular facet (the cranial end of the centrum of the first sacral vertebra), for articulation with the centrum of the last lumbar vertebra. Dorsad of this is the sacral canal, more depressed than the vertebral arch craniad of it. It supports a spinous process (Fig. 6, a) which is directed dorsad. At the junction of its lamina and radix is seen the prominent cranial articular process (b) with sometimes slight indications of a mammillary process on its lateral surface. Laterad of the articular facet is seen the cranial face of the expanded “pseudo-transverse process” (c) of the first sacral vertebra. The ventral border of the base is concave ventrad, forming an arc of about 120 degrees. The apex shows the caudal end of the last sacral centrum. Dorsad of this are the vertebral arch with a very short spinous process (a′), and the caudal articular processes (d). Laterad of the centrum appears the laterally directed thin transverse process (e).

Fig. 6.—Sacrum, Dorsal Surface.

Fig. 7.—Sacrum, Ventral Surface.

Fig. 6.—1, 2, 3, the three sacral vertebræ. a, a′, spinous processes; b, cranial articular process of first sacral vertebra; c, expanded transverse process of first sacral vertebra; d, caudal articular processes of third sacral vertebra; e, transverse processes of third sacral vertebra; f, tubercles formed by fused articular processes of the vertebræ; g, dorsal (or posterior) sacral foramina.

Fig. 7.—1, 2, 3, the three sacral vertebræ. a, the transverse ridges formed by the union of the centra; b, cranial articular processes of first vertebra; c, transverse process of first vertebra; d, caudal articular processes of third vertebra; e, transverse processes of third sacral vertebra; f, f′, ventral (or anterior) sacral foramina; g, notch which helps to form third ventral sacral foramen.

The ventral or pelvic surface (Fig. 7) is smooth, concave craniad, convex caudad, and crossed by two transverse ridges (a) along which are seen the ossified remains of the intervertebral fibro-cartilages. At the ends of the first ridge is a pair of nearly circular ventral (or anterior) sacral foramina (f) for[10] the passage of sacral nerves. At the end of the second ridge is a pair of ventral sacral foramina (f′), smaller than the first pair and continued laterocaudad into shallow grooves for the ventral rami of the sacral nerves. That portion of the bone lying laterad of a line joining the medial borders of these two pairs of foramina is known as the lateral mass of the sacrum and is composed of the fused transverse processes of the sacral vertebræ. At the caudal margin of the ventral surface there is a notch between the lateral mass and the centrum (g). When the caudal vertebræ are articulated, this notch helps to form a foramen for the third sacral nerve.

The dorsal surface (Fig. 6) is narrower at its cranial end than is the ventral surface. Its cranial border bears laterally a pair of cranial articular processes (b) with their medially directed facets and between them it is concave, so that a large dorsal opening is left into the vertebral canal between the last lumbar vertebra and the sacrum. Caudad of the articular processes are two pairs of tubercles (f). These are the fused cranial and caudal articular processes of the sacral vertebræ. Caudad of them are the caudal articular processes of the last sacral vertebra (d). Craniolaterad of the middle and cranial tubercles are dorsal (posterior) sacral foramina (g) for the transmission of the dorsal rami of the sacral nerves. Three spinous processes (a) appear between these rows of tubercles. They decrease in height caudad. That part of the surface included between the spinous process and the tubercles is made up of the fused laminæ of the sacral vertebræ. That part between the tubercles and a line joining the lateral margins of the dorsal (posterior) sacral foramina is formed by the fused radices of the sacral vertebræ.

The lateral surface may be divided into two parts. Craniad is a large rough triangular area with equal sides and with one of its angles directed ventrocraniad. It is the lateral face of the pseudo-transverse process of the first sacral vertebra (Fig. 6, c). A smooth curved surface (the auricular facet) along its ventral edge articulates with the ilium, while the dorsal portion is rough for attachment of ligaments. Caudad is the narrow longitudinal triangular area of the lateral faces of[11] the fused transverse processes of the second and third sacral vertebræ.

Caudal Vertebræ. Vertebræ caudales

(Fig. 1, y, and Figs. 8 and 9).—The caudal vertebræ (21-23 in number) decrease gradually in size to the last one. Caudad they become longer and more slender and lose the character of vertebræ. They become finally reduced to mere centra,—slender rods of bone knobbed or enlarged at their two ends (Fig. 8). The last one is more pointed than the others and bears at its caudal end a small separate conical piece, the rudiment of an additional vertebra.

Fig. 8.

Fig. 9.

Fig. 8.—Caudal Vertebra, from near the caudal end of the tail.

Fig. 9.—Fourth Caudal Vertebra, ventral view. a, transverse processes; b, cranial articular processes; c, hæmal processes; d, chevron bone.

The parts of a typical vertebra—vertebral arch, transverse processes, cranial and caudal articular processes—may be recognized in the vertebræ as far back as the eighth or ninth. The transverse processes (Fig. 9, a) are directed caudad and decrease rapidly in length. They are very small on the ninth vertebra, but may be recognized for a considerable distance back of this. The spinous process disappears at about the fourth caudal vertebra, and the vertebral canal becomes gradually smaller caudad, until on the eighth or ninth vertebra it becomes merely a groove open dorsad.

Caudad of the third vertebra for a considerable distance, each centrum bears on each lateral face at its cranial end a short anterior transverse process, and on its ventral face at its cranial end a pair of rounded tubercles, hæmal processes (c), which articulate with a small pyramidal chevron bone (d) so as to enclose a canal. These structures disappear caudad.

Cervical Vertebræ. Vertebræ cervicales

(Fig. 10).—The cervical vertebræ number seven. The first two of these are so peculiar as to require a separate description, so that the last five may be first considered.

Passing craniad from the fourth thoracic vertebra to the third cervical there is a gradual transition. The centra of the[12] cervical vertebræ are broader and thinner than those of the thoracic vertebræ, while the vertebral arches and vertebral canal are larger (Fig. 11). The caudal end of each centrum is concave and looks dorsocaudad when the centrum is held with its long axis horizontal. The cranial end of the centrum is convex and looks ventrocraniad when the centrum is horizontal. These peculiarities are more marked in the third vertebra than in the seventh. The spinous processes grow rapidly shorter as we pass craniad; the fifth, sixth, and seventh are directed dorsocraniad, the third and fourth dorsad.

Fig. 10.—Cervical Vertebræ, Side View.

a, spinous processes; b, cranial articular processes; c, caudal articular facet; d, intervertebral foramina; e, transverse process proper; f, processus costarius; g, wing of the atlas; h, dorsal arch of the atlas; i, atlantal foramen.

The caudal articular processes are situated at the junction of the radices and laminæ; their facets (Fig. 10, c) look ventrocaudolaterad. The cranial articular processes also become more prominent than is the rule in the thoracic vertebræ; they are borne at the junction of radix and lamina and have their facets (Fig. 11, b) directed dorsomediad. The cranial and caudal articular processes of each side are joined by a prominent ridge which is most pronounced in the third, fourth, and fifth vertebræ.

The characteristic feature of the cervical vertebræ is their transverse process, so called. In each of them it arises by two roots, one from the centrum and one from the arch.[13] These two roots, which are broad and thin, converge and unite so as to enclose a canal or foramen, the foramen transversarium (Fig. 11, g), for the vertebral artery. Laterad of the foramen the two parts of the process are, in the third cervical, almost completely united, the dorsal part being, however, distinguishable as a tubercle at the caudolateral angle of the thin plate formed by the process as a whole. This dorsal component is the transverse process proper (Figs. 10 and 11, e), while the ventral portion represents a rib, and is hence known as the processus costarius (f). The expanded plate formed by the union of these two processes is directed nearly ventrad and somewhat craniad in the third, fourth, and fifth vertebræ. The two components of the process gradually separate as we pass caudad; in the fourth and fifth vertebræ the part which represents the transverse process proper forms a very prominent tubercle at the caudolateral angle of the plate formed by the processus costarius. In the sixth (Fig. 11) the two parts are almost completely separated; the dorsal part forms (e) a slender knobbed process, while the processus costarius is divided into two portions (f and f′) by a broad lateral notch. In the seventh the ventral part (processus costarius) is usually quite lacking, though sometimes represented by a slender spicule of bone. In the former case the foramen transversarium is of course likewise lacking.

Fig. 11.—Sixth Cervical Vertebra, Cranial End.

Fig. 12.—Atlas, Ventral View.

Fig. 11.—a, spinous process; b, cranial articular facet; c, lamina; d, radix or pedicle; e, transverse process proper; f, f′, processus costarius; g, foramen transversarium; h, centrum; i, vertebral canal.

Fig. 12.—a, ventral arch; b, tuberculum anterius; c, lateral masses; d, transverse processes; e, cranial articular facets; f, groove connecting the foramen transversarium with the atlantal foramen; g, atlantal foramen; h, caudal articular facets.

[14]

The Atlas (Fig. 10, 1; Fig. 12).—The first cervical vertebra or atlas has somewhat the form of a seal ring. The centrum is absent; it has united with the second vertebra to form the odontoid process or dens. Its place is taken in the atlas by a narrow flat arch of bone, narrower at the ends than in the middle, the ventral arch (Fig. 12, a) of the atlas. This connects the lateral, thicker portions of the ring ventrally and bears on its caudal margin a blunt tubercle (tuberculum anterius, Fig. 12, b). Laterally the ring is thickened, forming thus the lateral masses (c) which are continued into the broad thin transverse processes (Fig. 10, g; Fig. 12, d). Each lateral mass bears at its cranial end on its medial surface a concave, pear-shaped facet, cranial (or superior) articular facet, (Fig. 12, e) for articulation with the condyles of the skull. These facets look craniomediad. Dorsad of each is a foramen, the atlantal foramen (Fig. 10, i; Fig. 12, g), which pierces the dorsal arch at its junction with the lateral mass. Caudal to the facet, on the medial face of each lateral mass, within the vertebral canal, is a tubercle. To the two tubercles are attached the transverse ligament (Fig. 14, b) which holds in place the odontoid process (dens) of the axis.

That part of the lateral mass which bears the articular facet projects craniad of the dorsal arch and is separated by a deep triangular notch from the transverse process. Along the bottom of this notch runs a groove (Fig. 12, f), convex craniad, which connects the cranial end of the foramen transversarium and the atlantal foramen. The vertebral artery passes along it. The foramen transversarium is circular. It is bounded laterally by the lateral masses, and dorsally by the dorsal arch.

The dorsal arch (Fig. 10, h) is two to three times as broad as the ventral, has a thick convex cranial border with a median notch, and a thin concave caudal border.

Fig. 13.—Axis or Epistropheus, Side View.

a, odontoid process or dens; b, cranial articular facets; c, spinous process; d, caudal articular facet; e, transverse process; f, foramen transversarium.

The caudal articular facets (Fig. 12, h) are borne by the caudal ends of the lateral masses. They are slightly concave, triangular, and look caudomediad, so that their dorsal borders form with the caudal border of the dorsal arch nearly a semicircle. The transverse processes are flat and directed laterad.[15] The attached margin of each is about two-thirds the length of the thinner free margin. The somewhat thicker caudal end of the transverse process projects further caudad than any other part of the vertebra and is separated by a slight notch from the caudal articular facet. From the bottom of this notch the foramen transversarium extends craniad and opens at the middle of the ventral face of the transverse process.

Epistropheus or Axis (Fig. 10, 2; Fig. 13).—The second cervical vertebra (epistropheus or axis) is not so wide as the atlas but is much longer. Craniad the centrum is continued into a slender conical, toothlike projection, the dens or odontoid process (Fig. 13, a) which represents the centrum of the atlas. The dens is smooth below for articulation with the ventral arch of the atlas. It is rougher above. Laterad of the dens the centrum bears a pair of large cranial articular facets (b) which look craniolaterad. These have each the form of a right-angled triangle with rounded angles, one side of the triangle being nearly horizontal. Each is separated from the articular face of the dens by a roughened groove. The spinous process (c) runs the length of the vertebral arch. It extends craniad of the vertebral arch nearly as far as the dens, as a flat rounded projection. Caudad of the vertebral arch it projects for a short distance as a stout triangular spine. The caudal articular facets (d) are borne on thickenings of the caudolateral portions of the arch; they face almost directly ventrad. The transverse process (e) is slender and triangular and directed nearly caudad. Its apex reaches no farther than the caudal or articular face of the centrum. Its base is traversed by the foramen transversarium (f).

Differential Characters of the Cervical Vertebræ.—It is possible to identify each of the cervical vertebræ:

The first by the absence of the centrum.

The second by the dens or odontoid process.

The third by the small spinous process and slightly marked[16] tubercle of the transverse process, and by a median tubercle on the cranial border of the vertebral arch.

The fourth by the spinous process directed dorsad, and the short thick tubercle of the transverse process not trifid.

The fifth by the spinous process directed craniad, and the more slender spine-like tubercle of the transverse process not trifid.

The sixth by the trifid transverse process.

The seventh by the long spinous process and the slender simple transverse process, and by the usual absence of the foramen transversarium.

LIGAMENTS OF THE VERTEBRAL COLUMN.

Fibro-cartilagines intervertebrales.—The separate vertebræ (except the atlas and axis) are united by the disk-shaped intervertebral fibro-cartilages, which are situated between the centra of the vertebræ. Each consists of a central pulpy portion and a fibrous outer portion, covered by strong intercrossing tendinous fibers which unite with the periosteum of the vertebræ.

Ligamentum longitudinale anterius.—On the ventral face of the centra of the vertebræ, from the atlas to the sacrum, lies a longitudinal ligament, the anterior longitudinal ligament. It is very small, almost rudimentary, in the cervical region: large and strong in the thoracic and lumbar regions.

Ligamentum longitudinale posterius (Fig. 14, a).—A corresponding ligament (posterior longitudinal ligament) lies on the dorsal surface of the centra (therefore within the vertebral canal). It is enlarged between each pair of vertebræ and closely united to the intervertebral fibro-cartilages.

Ligamentum supraspinale.—Between the tips of the spinous processes of the thoracic and lumbar vertebræ extend ligamentous fibers. They are not united to form a distinct band, and can hardly be distinguished from the numerous tendinous fibers of the supraspinous muscles. Together they represent the supraspinous ligament. From the tip of the spinous process of the first thoracic vertebra to the caudal end of the spine of the axis extends a slender strand representing[17] the ligamentum nuchæ or cervical supraspinous ligament. It is imbedded in the superficial muscles of this region, some of which take origin from it.

Ligamentous fibers are also present between the spinous processes of the vertebræ (ligamenta interspinalia): between the transverse processes (ligamenta intertransversaria), and between the vertebral arches (ligamenta flava).

Capsulæ articulares.—The joints between the articular processes are furnished with articular capsules attached about the edges of the articular surfaces. These are larger and looser in the cervical region.

Atlanto-occipital Articulation.—The joint between the atlas and the occipital condyles has a single articular capsule, which is attached about the borders of the articular surfaces of the two bones. This capsule is of course widest laterally, forming indeed two partially separated sacs, which are, however, continuous by a narrow portion across the ventral middle line. This capsule communicates with that which covers the articular surface of the dens, and through this with the capsule between the atlas and axis. That portion of the capsule which covers the space between the ventral arch of the atlas and the occipital bone represents the anterior atlanto-occipital membrane; it is strengthened by a slender median ligamentous strand. The posterior atlanto-occipital membrane covers in the same way the space between the dorsal arch of the atlas and the dorsal edge of the foramen magnum. In it a number of different sets of fibers, with regard to direction and to degree of development, may be distinguished; these have sometimes been considered separate ligaments.

The lateral ligaments of the atlas begin at the lateral angle of the cranial margin of the atlas, at about the junction of its dorsal and ventral arches, and pass cranioventrad to the jugular processes.

Articulation between the Axis and Atlas.—The articular capsule is large and loose, being attached to dorsal and ventral borders of the atlas, about the articular surfaces of the axis, and to the cranial projection of the spine of the atlas. It also passes craniad along the ventral side of the dens and communicates[18] here with the capsule of the atlanto-occipital articulation. In the dorsal part of the capsule a short strong ligamentous strand is developed, connecting the caudal border of the dorsal arch of the atlas with the tip of the cranial projection of the spinous process of the axis.

Fig. 14.—Ligaments of the Odontoid Process or Dens.

First three cervical vertebræ and base of the skull, with dorsal surface removed. a, ligamentum longitudinale posterius; b, transverse ligament of the atlas; c, ligamenta alaria; d, odontoid process; e, occipital condyles; 1, 2, 3, the first three cervical vertebræ; 4, basal portion of the occipital bone.

The dens or odontoid process is held in place by the transverse ligament (Fig. 14, b) of the atlas, which passes across the process as it lies within the vertebral canal of the atlas. The transverse ligament is attached at its two ends to the medial surface of the sides of the atlas at about the region where the dorsal and ventral arches of the atlas unite.

From the cranial end of the odontoid process the two ligamenta alaria (Fig. 14, c) diverge craniolaterad to the rough ventromedial angle of the condyles of the occipital bone.

II. RIBS. COSTÆ (Figs. 1 and 15.)

The cat has thirteen pairs of ribs. One of the fifth pair (Fig. 15) may be taken as typical. It is a curved flattened rod of bone attached at its dorsal end to the vertebral column, and at its ventral end to a cartilage (costal cartilage, Fig. 15, f) which serves to unite it to the sternum.

The most convex portion of the bone is known as the angle (e). Each rib presents a convex lateral and a concave medial surface, a cranial and a caudal border. The borders are broad dorsad and narrow ventrad, while the surfaces are narrow dorsad and broad ventrad. The rib has thus the appearance of having been twisted.

The rib ends dorsad in a globular head or capitulum (a), by which it articulates with the costal demifacets of two contiguous thoracic vertebræ. Between the capitulum and angle[19] on the lateral surface is an elevated area, the tubercle, marked by the smooth tubercular facet (c) for articulation with the transverse process of a vertebra. The constricted portion between the head and tubercle is known as the neck (collum) (d). The angle is marked by a projecting process (e) (angular process) on its lateral border, for attachment of a ligament.

Fig. 15.—Fifth Rib of Left Side, Cranial View.

a, head; b, tubercle; c, tubercular facet; d, neck; e, angle, with angular process; f, cartilage.

The ribs increase in length to the ninth (the ninth and tenth are of the same length) and then decrease to the last. They decrease in breadth behind the fifth. The first is nearly in a dorsoventral plane, while the others have their dorsal ends inclined slightly craniad.

Fig. 16.—Sternum, Ventral View.

a, manubrium; b, the separate pieces forming the body; c, bony part of the xiphoid process (the expanded cartilaginous portion not being shown); d, facet for attachment of first rib.

The tubercles become less prominent as we pass caudad and are absent on the last two or three ribs, which do not articulate with the transverse process.

The first nine ribs (true ribs or costæ veræ) are attached separately to the sternum by their costal cartilages. The last four (false ribs or costæ spuriæ) are not attached separately to the sternum. The costal cartilages of the tenth, eleventh, and twelfth are united to one another at their sternal ends. They may be united also to the ninth costal cartilage or to the sternum by a common cartilage of insertion, or they may be quite free from the sternum. The thirteenth costal cartilages are free (floating ribs).

Ligaments of the Ribs.—The articular surfaces between the head of the rib and the centra, and between the tubercle and the transverse process of the vertebra, have each an articular capsule. There are also a number of small ligamentous bands from the tuberosity and the neck of the rib to the transverse process of the vertebra.

[20]

III. STERNUM. (Fig. 16.)

The sternum consists of three portions, a cranial piece or manubrium (a), a caudal piece or xiphoid process (c), and a middle portion or body (corpus), which is divided into a number of segments (b).

To the sternum are united the ventral ends of the first nine ribs. It thus forms the median ventral boundary of the thorax. Since the thorax decreases in dorsoventral measurement craniad, the long axis of the sternum is inclined from its caudal end dorsocraniad, and if continued would strike the vertebral column in the region of the first cervical vertebra.

The manubrium (a) makes up about one-fifth the whole length of the sternum and projects craniad of the first rib. It has the form of a dagger and presents a dorsal surface and two lateral surfaces, the latter uniting ventrad to form a sharp angle. In the middle of the lateral surface near the dorsal margin is an oval articular surface (d) borne on a triangular projection. It looks caudodorsad and is for the first costal cartilage.

The caudal end articulates with the body by a synchondrosis and presents a slightly marked oval facet on each side for the second costal cartilage.

The body consists of six cylindrical pieces (b) enlarged at their ends and movably united by synchondroses. They increase in breadth from the first, and decrease slightly in length and thickness. At the caudal end of each near its ventral border there is a pair of facets looking caudolaterad. They are for the costal cartilages.

The xiphoid process (c) is a broad thin plate of cartilage at its caudal end; bony and cylindrical[21] at its cranial end. It is attached by its base to the last segment of the body by a considerable cartilaginous interval, while the opposite end is free and directed caudoventrad.

The cartilage of the ninth rib is attached to the lateral face of the cartilage between the xiphoid and the body, and just caudad of this the common cartilage of insertion of the tenth, eleventh, and twelfth costal cartilages is attached, if present.

IV. THE SKULL.

The bones of the head consist of the skull proper together with a number of separate bones forming part of the visceral skeleton; these are the lower jaw, the hyoid, and the ear-bones.

The skull proper is considered as divided into cranial and facial portions. The former includes all the bones which take part in bounding the cranial cavity or cavity of the brain; the latter includes the bones which support the face.

The cranial portion of the skull includes all that part enclosing the large cavity which contains the brain. For convenience this portion may be considered as made up of three segments, each of which forms a ring surrounding a part of the cranial cavity. The first or caudal segment or ring consists of the occipital bone (with the interparietal) surrounding the foramen magnum. The second segment consists of the sphenoid ventrad, the parietals laterad and dorsad. Between the first and second segments are intercalated laterally the temporal bones containing the auditory organ. The third segment or ring consists of the presphenoid ventrad, of the frontals laterad and dorsad. The cranial opening of this ring is closed by the lamina cribrosa of the ethmoid.

The cranial portion of the skull therefore contains eleven separate bones: one occipital, one interparietal, two temporals, one sphenoid, two parietals, one presphenoid, two frontals, and one ethmoid.

The facial portion of the skull is much smaller than the[22] cranial, and lies craniad of the latter; it encloses the nasal cavity. It contains the following thirteen bones: two palatines, one vomer, two maxillaries, two lachrymals, two premaxillaries, two nasals, two malar or zygomatic bones. The two halves of the mandible or lower jaw are frequently included in the facial portion of the skull, making in all fifteen separate bones in this part of the skull.

In the following the bones of the skull are first described separately, then an account is given of the skull as a whole.

Occipital Bone. Os occipitale

(Figs. 17 and 18).—The occipital bone forms the most caudal portion of the cranium, entering into the formation of its caudal wall and of its base. It connects the cranium with the vertebral column and surrounds a large opening, the foramen magnum (d), by means of which the cranial cavity communicates with the vertebral canal.

Fig. 17.—Occipital Bone, Caudal or Outer Surface.

Fig. 18.—Occipital Bone, Inner Surface.

a, basilar portion; b, lateral portions; c, squamous portion; d, foramen magnum; e, occipital condyles; f, jugular processes; g, jugular notch; h, lambdoidal ridge; i, external occipital crest; j, external occipital protuberance.

In young kittens four portions may be distinguished in this bone. These are, a basal portion (the basioccipital bone), two lateral portions (the exoccipital bones), and a dorsal portion (the supraoccipital bone). These four bones remain separate through life in many lower vertebrates. In the adult cat they are completely united into a single bone, but it is convenient to describe this bone as made up of four parts: a basal portion (a) (pars basilare), corresponding to the basioccipital, two lateral portions (b) (partes laterales), corresponding to the exoccipitals, and a squamous portion (c) (squama occipitalis) corresponding to the supraoccipital.

[23]

The basilar portion (a) as seen from the dorsal or ventral surface is oblong and flattened. It is broadest at the junction of its middle and last thirds, and tapers toward both ends. It presents a cranial end and a caudal end, a dorsal, a ventral, and two lateral surfaces. Its caudal end, which helps to form the ventral boundary of the foramen magnum, is concave from side to side. The cranial end is transversely elongate, about five times as broad as high, pointed laterally and roughened for attachment to the caudal end of the body of the sphenoid, which has a corresponding form. The joint is a synchondrosis.

The dorsal surface is concave from side to side, forming a longitudinal groove in which rest the pons and medulla. The concavity is more pronounced caudad, where the bone is thinner at its middle.

The ventral surface is marked by three parallel longitudinal ridges. One of them is median and expands caudad into a smooth triangular elevated area which extends to the foramen magnum. The other two ridges run near the lateral edges of the bone. Between them and the median ridge the surface is smooth and is depressed caudad. Laterad of each lateral ridge is a rough triangular surface overlaid in the natural state by the medial edge of the tympanic bulla.

The lateral surfaces are smooth and sharp and abut against the petrous portion of the temporal bone. They pass into the lateral portions of the occipital caudad.

The lateral portions (b) of the occipital arise from the caudal margin of the basilar portion in the transverse plane; a short distance laterad of the line of junction they turn dorsad at an angle of nearly ninety degrees. They form the lateral boundaries of the foramen magnum and pass dorsally into the squamous portion.

The external surface of each presents an elongated elevated spirally curved surface, the occipital condyle (c), for articulation with the atlas. A small part of each condyle is formed from the basal portion. The two condyles are separated from one another ventrally by a narrow notch, and each extends (laterad) along the border of the foramen magnum to a point slightly dorsad of the transverse diameter of the latter.

[24]

Laterad of each condyle the bone is elevated into a blunt triangular projection, the jugular process (f), which covers the caudal end of the tympanic bulla. Between the jugular process and the condyle is a deep depression.

The internal surface (Fig. 18) is concave dorsoventrally, following the outline of the foramen magnum. It is convex from side to side. It is smooth except at its outer margin, which is rough for articulation with the mastoid portion of the temporal bone. The cranial face of the jugular process (f) shows a rough concavity for the reception of the bulla tympani. Mediad of the jugular process is a notch (jugular notch) (g) which when the bones are articulated forms part of the boundary of the jugular foramen. Mediad of this notch is a foramen which forms one end of the hypoglossal canal. It passes dorsocaudad into the cranial cavity and transmits the hypoglossal nerve. Dorsad of the hypoglossal canal is the cranial opening of the condyloid canal, which passes caudad and opens just craniad of the dorsal end of the condyle. It transmits a vein. The outer border of this portion is rough for articulation with the petrous and mastoid portions of the temporal.

The squamous portion (c) has the form of a sector of a circle whose arc is a little more than ninety degrees. The central angle of the sector is truncated and bounds the foramen magnum dorsally. The arc of the sector forms the dorsal margin of the bone, while along the radii it passes into the lateral portions. Its dorsal portion is thick and porous; its ventral portion near the foramen magnum is thin and compact.

The external surface (Fig. 17) is marked by a prominent ridge, the lambdoidal ridge (h), parallel with the dorsal border and near to it. The narrow portion of the outer surface that lies dorsad of the ridge forms an angle of about ninety degrees with the remainder of the surface.

A median crest (i) extends ventrad from the middle of the lambdoidal ridge toward the foramen magnum; this is the external occipital crest. At its junction with the lambdoidal ridge it is elevated into a tubercle, the external occipital protuberance (j).

[25]

The inner surface (Fig. 18) presents depressions for the convolutions of the cerebellum.

The dorsal border is thick and rough for articulation with the parietals and interparietal. The ventral border abuts on the foramen magnum and is thin and smooth.

Interparietal Bone. Os interparietale

(Fig. 19).—This is a small triangular bone lying between the parietals, with its apex directed craniad, and its base in contact with the squamous portion of the occipital.

Its dorsal surface is arrow-shaped and has its posterior border notched. It is marked by a median crest (part of the sagittal crest) which is continued craniad from the middle of the lambdoidal crest.

The ventral surface is irregularly triangular, smooth, and concave. The three borders are rough for articulation with the parietals and occipital.

Fig. 19.—Interparietal Bone, Outer Surface.

Fig. 20.—Sphenoid Bone, Dorsal or Internal Surface.

Fig. 19, showing the sagittal crest running craniocaudad across its middle.

Fig. 20.—a, body; b, wings; c, pterygoid process; d, tuberculum sellæ; e, dorsum sellæ; f, sella turcica; g, notch which aids in forming the foramen lacerum; h, longitudinal groove of alisphenoid; i, notch which aids in forming the orbital fissure; j, foramen rotundum; k, foramen ovale.

Sphenoid Bone. Os sphenoidale

(Fig. 20).—The sphenoid bone of man is represented in the cat by two entirely distinct bones,—one cranial, the other caudal. The cranial portion may be designated as the presphenoid (Fig. 21); the caudal part will be described as the sphenoid (proper) (Fig. 20).

The sphenoid bone in the kitten is in three parts: a central portion, the basisphenoid, and two lateral portions, the alisphenoids. In many lower vertebrates these three bones are permanently distinct, but in the adult cat they are united to form the sphenoid bone. To these there is added a fourth element, separate in many vertebrates as the pterygoid bone.[26] The sphenoid may thus be described as composed of a central portion, the body (a) (basisphenoid), and of two thin expanded wings (b) (alisphenoids, alæ magnæ of the human sphenoid); each of which has arising from it a thin curved process, the pterygoid process (c), directed craniad and largely made up of the pterygoid bone.

The body of the sphenoid (a) lies in the middle line of the base of the skull. It is wedge-shaped, with the converging sides of the wedge directed laterad and its apex pointed craniad.

It has six surfaces, of which the dorsal and a part of the laterals look into the cranial cavity. The cranial end articulates with the body of the presphenoid, and the caudal with the body of the occipital.

The dorsal surface is triangular, with one apex of the triangle truncated, elevated, and directed craniad. This elevation is the tuberculum sellæ (d). Just caudad of the middle the surface presents a rectangular elevation with rounded angles, the dorsum sellæ (e). The cranial end of the dorsum sellæ presents at each dorsolateral angle a very small smooth tubercle which represents one of the posterior clinoid processes of man. Between this elevation and the elevated cranial end of this surface there is a deep excavation, the sella turcica (f), in which in the natural state is lodged the hypophysis. Near the cranial end of the sella is a small foramen, probably nutrient. At the caudal end of the body a slight notch (g) separates it from the wing: this notch forms a part of the foramen lacerum. Against this notch fits the apex of the petrous bone, and from it a groove (carotid groove) is continued mediocraniad to the sella turcica.

The ventral surface (Fig. 41, 3) is triangular, smooth, and nearly flat; it is marked by a median ridge which is the continuation craniad of the ridge on the ventral face of the basilar portion of the occipital.

Its caudal angles are separated from the rest of the bone by sharp triangular elevations, laterad of which are rough triangular areas, overlaid when the bones are articulated by a triangular spine from the tympanic bulla.

[27]

Its lateral surfaces are mostly covered by the wings. They appear at the sides of the elevated cranial end of the dorsal surface as triangular areas.

The caudal end is concave, rough, and has the form of the cranial end of the basilar part of the occipital.

The cranial end is nearly square and rough for articulation with the body of the presphenoid.

The Wing (alisphenoid; ala magna of the human sphenoid) (Fig. 20, b).—This is a thin quadrilateral plate of bone attached by its medial border to nearly the whole of the lateral surface of the body. Its middle portion lies nearly in the same plane as the body, but its ends are curved dorsad so that its internal surface is concave and its external surface is convex. The curvature is most pronounced near the long lateral border, so that this border forms nearly a semicircle.

The internal surface supports the occipital lobe of the cerebrum. It is marked by a rounded groove (h) which is parallel with the lateral surface of the body. The dorsal margin of the groove projects mediad in the form of a sharp ridge which is broadest caudad, where it often reaches nearly to the posterior clinoid process. The groove passes craniad into three foramina. The first (cranial) of these, the orbital fissure (i), is large and lies between the wing, the body, and the pterygoid process. It is incomplete, but is completed by the presphenoid. The second foramen is small and rounded; it is the foramen rotundum (j). The third, foramen ovale (k), is larger and oval and penetrates the wing through about the middle of its longitudinal axis. Another minute foramen penetrates the sphenoid between the wing and the body of the bone, just laterad of the tuberculum sellæ. This foramen is continuous craniad with a groove on the dorsal surface of the pterygoid process; the groove and foramen constitute the pterygoid canal. It transmits a nerve.

The external surface shows the orbital fissure, the foramen rotundum and the foramen ovale, bounded ventrally by a sharp ridge, which is continued onto the pterygoid process. Between this ridge and the body the surface is longitudinally grooved for the tuba auditiva or Eustachian tube.

[28]

The semicircular margin of the bone articulates with the squamous portion of the temporal. At the junction of its caudal and middle third there is sometimes a toothlike projection which underlies the root of the zygoma.

The whole of the cranial margin, except the lateral end, articulates with the wing of the presphenoid. At this end the angle formed by the junction of lateral and cranial borders is produced into a flat process, which passes dorsocaudad between the squamous portion of the temporal and the frontal, and articulates by the roughened internal surface of its free end with a similar process from the parietal.

The caudal margin laterad of the groove is bevelled and roughened at the expense of the dorsal surface and is overlaid by the ventral end of the tentorium. Mediad of the groove it projects caudad as a slender point, the lingula of the sphenoid. This is received into a narrow cleft between the apex of the petrous bone and the bulla tympani.

The pterygoid process (c) is a nearly square, thin plate of bone. The medial surface is smooth and concave, the lateral face is convex and marked by two parallel ridges. The medial one of these is continued craniad from the bony septum which separates the orbital fissure from the foramen rotundum, and the lateral one from the septum which separates the foramen rotundum from the foramen ovale. A sharp triangular spine projects laterad from near the caudal end of the lateral ridge.

The two ridges and that part of the lateral surface of the bone included between them form a part of the sphenoid bone known as the pterygoid process of the sphenoid bone, in those cases where the pterygoid is a separate bone.

The remainder of the process is equivalent to the pterygoid bone of other vertebrates.

Between the caudal margin of this bone and the lateral of the two ridges, i.e., between the pterygoid bone and the pterygoid process of the sphenoid, is a long deep fossa, the internal pterygoid fossa (Fig. 40, s). The laterocaudal margin of the pterygoid process projects caudad, as a curved triangular spine, the hamulus or hamular process (Fig. 40, t; Fig. 43, i).

[29]

The Presphenoid Bone. Os presphenoidale

(Fig. 21).—In a young cat this bone is in three pieces, a basal portion (presphenoid) and two wings (orbitosphenoid bones). These bones remain distinct throughout life in many lower vertebrates, but in the adult cat they fuse to form a single bone. We may nevertheless conveniently describe this bone as made up of a body (a) (the basisphenoid), and two wings (b), the orbitosphenoids (the alæ parvæ of the human sphenoid).

Fig. 21.—Presphenoid Bone, Ventral View.

a, body; b, wings; c, optic foramina.

The body (a) lies in the base of the skull in the median line, craniad of the basisphenoid. It has the form of a rectangular prism about twice as long as broad. It is hollow, and the cavity is divided by a median longitudinal partition into two cavities (sphenoidal sinuses, Fig. 43, l). The sphenoidal sinuses are continued craniad into the cavities of the ethmoid. The body has six surfaces:

The dorsal or internal surface (Fig. 42, n) looks into the cranial cavity and is continuous with the dorsal surface of the wings. The caudal end of the body is depressed, and when united to the basisphenoid aids in forming the cranial wall of the sella turcica. At each caudolateral angle is a short spine, the anterior clinoid process. At about one-third the length of the bone from the caudal end is a transverse groove (chiasmatic groove, Fig. 42, m) for the optic chiasma. Its ends lead into two round foramina (the optic foramina, Fig. 42, l; Fig. 21, c) which pass craniolaterad between the body and the wings of the presphenoid and transmit the optic nerve and the ophthalmic artery.

The ventral surface (Fig. 21) is hour-glass-shaped and marked by a smooth median ridge, continuous with the ridge on the basisphenoid and overlaid at its cranial end by the vomer. The caudal end presents a rough triangular area on each side, for articulation with the pterygoid process of the sphenoid bone, while the cranial end has similar areas overlaid by the nasal portion of the palatine bones.

The lateral surface looks towards the orbitotemporal fossa. It is notched near the caudal end by the ventral border of the[30] optic foramen (c). Caudad of this foramen the surface presents an oblique groove which forms in the natural condition the medial boundary of the orbital fissure.

Craniad of the optic foramen the surface is smooth and marked near its ventral border by a longitudinal ridge which forms part of the dorsal boundary of a fossa, the external pterygoid fossa (Fig. 40, p).

The caudal end presents ventrally a quadrangular rough surface for articulation with the body of the sphenoid. The cranial end presents the two sphenoidal sinuses separated by a median partition.

The median partition articulates by its free border with the lamina perpendicularis of the ethmoid. At its ventral end is the abruptly truncate end of the median ridge of the ventral surface, which is continuous with the ventral cartilaginous portion of the lamina perpendicularis. The lateral walls of the sphenoidal sinuses are continued craniad of the dorsal and ventral walls and of the median partition, and articulate ventrally with the nasal portion of the palatine bones, and dorsally with the orbital plate of the frontal. Between them is received the caudal ends of the labyrinths of the ethmoid in the middle, while between their dorsal edges is received the caudal end of the cribriform plate, and between their ventral edges the expanded end of the vomer.

The wings (b) arise each from nearly the whole of the dorsolateral angle of the body. They form prominent nearly horizontal triangular projections over the optic foramina.

The dorsal and ventral surfaces are smooth and continuous respectively with the dorsal and lateral surfaces of the body. The dorsal surface looks into the cranial cavity, while the ventral looks into the orbitotemporal fossa.

Craniad of the apex of the wing its border articulates with the ventral edge of the orbital portion of the frontal bone. Caudad of the apex the border articulates with the cranial border of the wing of the sphenoid.

Temporal Bone. Os temporale

(Figs. 22 and 23).—This forms a part of the lateral wall of the cranium, filling the gap between the occipital and the sphenoidal segments. It is made[31] up of three portions which are distinct in kittens but somewhat firmly united in adult cats. In lower vertebrates these portions are distinct bones called the Squamous, the Petrous, and the Tympanic bones. In the cat they may be described as the squamous (a), petrous (b), and tympanic (c) portions of the temporal bone.

Fig. 22.—Temporal Bone, External Surface.

Fig. 23.—Temporal Bone, Internal Surface.

a, squamous portion; b, petrous portion; c, c′, tympanic portion (c, entotympanic; c′, ectotympanic); d, zygomatic process; e, mastoid portion of the petrous; f, mandibular fossa; g, postmandibular process; h, tuberculum articulare; i, external auditory meatus; j, stylomastoid foramen; k, pit for tympanohyal bone; l, mastoid process; m, grooves bounding the jugular foramen; n, internal auditory meatus; o, appendicular fossa; p, hiatus facialis; q, styliform process of tympanic bulla.

The squamous portion (a) (squama temporalis) is thin and oval or has the form of an equilateral triangle with rounded angles, with a curved process, the zygomatic process (d), arising from its ventral border. Its outer surface (Fig. 22) is convex and smooth and gives origin to part of the temporal muscle. Its inner surface (Fig. 23) is concave and smooth except near the margins, where it is bevelled and rough. The roughened border is broader dorsad and caudad. The ventral margin of the bone is turned mediad at its cranial end so that the lateral face of the inverted portion looks ventrad. By this portion of its lateral face the squamous rests upon the tympanic bulla, and its edge articulates with the tentorium and the wing of the sphenoid. The remaining (caudal) portion of the ventral border overlies the mastoid portion (e) of the petrous. By the remainder of its border the bone articulates with the parietal dorsad and with the wing of the sphenoid craniad. The[32] roughened portion of its inner surface overlies the margins of both these bones.

The zygomatic process (d) is formed by the confluence of two roots. One of these starts from the ventral end of the lambdoidal ridge and passes along the ventral margin of the squama dorsad of the external auditory meatus. The other arises abruptly from the cranioventral angle of the bone. The process thus formed is at first broad and passes horizontally laterad and slightly craniad. It soon grows more slender and turns gradually craniad, while at the same time it twists so that the surface which is dorsal at the base becomes medial at the tip; the posterior root which is continuous with the caudal border at the base is continuous with the dorsal border at the apex. On the ventral surface of the base is a transversely elongated concave articular surface, the mandibular fossa (f), for the condyloid process of the lower jaw. Caudad of this is a sharp transverse ridge, the postmandibular or postglenoid process (g), and craniad of the lateral end of the fossa a slight tubercle, the tuberculum articulare (h). Near its apex the zygomatic process is more slender and its ventral border is bevelled for articulation with the malar or zygomatic bone.

The tympanic (Figs. 22 and 23, c; Fig. 24) is expanded into a large hollow olive-shaped bone which is known as the auditory bulla and encloses the tympanic cavity. Its substance is very compact. Unlike the tympanic of most other mammals it is developed from two bones, known as the ectotympanic (Fig. 22, c′) and entotympanic (c). These are strongly marked in young kittens, and can usually be easily distinguished in adult cats. The entotympanic (Figs. 22 and 23, c) forms the larger part of the bulla, constituting its ventral and medial surfaces; it is thin, smooth, and transparent. The ectotympanic (Fig. 22, c′) surrounds the external auditory meatus: it is thicker and more opaque than the entotympanic. The bulla lies ventrad of the squamous, and in an external view conceals a large part of the petrous.

On its lateral surface it presents near the dorsal border an irregular oval opening, that of the external auditory meatus (Fig. 22, i), which leads into the tympanic cavity. Caudad of[33] the external auditory meatus is a nearly dorsoventral groove, which, when the bone is articulated, forms a part of the boundary of the stylomastoid foramen (Fig. 22, j); just ventrad of this groove is a pit (k) which lodges the tip of the tympanohyal bone.

Craniad the bone is produced into a short spine, the styliform process (q), which lies in a horizontal groove in the ventral surface of the basisphenoid. Laterad of this spine is a groove for the tuba auditiva or Eustachian tube.

Fig. 24.—Tympanic Bulla, Isolated, Medial Surface.

a, inner end of auditory meatus; b, partition dividing tympanic cavity; c, styliform process.

The medial surface (Fig. 24) presents in the middle near its ventral margin a short triangular spine which lies in the natural state against the ventral surface of the basilar portion of the occipital.

Caudad of this spine the surface is marked by two or three vertical parallel grooves (Fig. 23, m). They indicate the portion of the bone which bounds the jugular foramen, and are possibly impressions of the ninth, tenth, and eleventh nerves.

The dorsal two-thirds of the medial surface is lacking in the disarticulated bulla (Fig. 24), so that the cavity of the bone is exposed. This opening is in the natural state closed by the petrous bone. The caudal end is rough where it is overlaid by the jugular process.

On the inner surface of the tympanic bulla is seen the thickened margin of the inner end of the auditory meatus (Fig. 24, a). To it is attached the membrana tympana. In the median dorsal line this margin is notched for the reception of the incus and head of the malleus. From the lateral wall of the cavity at the line of junction of the ectotympanic and entotympanic a thin bony partition (b) rises. It runs almost directly mediad; is concave dorsally and divides the tympanic cavity into two chambers.

The Petrous Portion (Fig. 23, b, and Fig. 25).—This consists of two parts, a very dense part (the petrous portion proper, Fig. 25), which has the form of a triangular pyramid and encloses the auditory labyrinth, and a less dense part, the[34] mastoid portion (Figs. 22 and 23, e), which is flattened and triangular and is attached by its base to the base of the pyramid.

The petrous portion may be described as having a base and three sides, lateral, dorsal, and medial. It completes the medial wall of the tympanic bulla, so that it is not possible to see it from the exterior of a skull except through the auditory meatus (Fig. 22, i). When the bones of the skull are articulated its dorsal surface is covered by the tentorium and alisphenoid. Its lateral face looks into the tympanic cavity, while the medial face looks into the cranial cavity.

Fig. 25.—Petrous Bone of Right Side, Lateral Surface, Enlarged.

a, fenestra cochleæ; b, promontory; c, fenestra vestibuli; d, fossa for the tensor tympani muscle; e, fossa for incus and malleus; f, fossa continuous with stylomastoid foramen; g, foramen leading to facial canal.

Its lateral face (Fig. 25) (medial wall of the tympanic cavity) presents just ventrad of the middle of its base a large circular foramen, the fenestra cochleæ (a) (or fenestra rotunda), which looks caudolaterad; it leads into the cochlea. The fenestra cochleæ lies at the summit of a nipple-like elevation, the promontory (b), which is continued toward the apex of the bone as a gradually diminishing semicylindrical ridge, due to the presence within it of the bony cochlea. Dorsad of the fenestra cochleæ is the much smaller fenestra vestibuli (c) (or ovalis) which leads into the vestibule. It is occupied in the natural condition by the base of the stapes.

Dorsocraniad of the fenestra vestibuli is a large fossa (d) which contains the tensor tympani muscle. Dorsocaudad of this, partly bounded by the squamous portion of the temporal, is another large fossa (e), the cranial end of which is occupied by the incus, while its caudal end is occupied by the head of the malleus.

Nearly caudad of this fossa and separated from it by an oblique bony septum is a third fossa (f) which is narrow and curved. It is continuous with a notch in the mastoid portion of the bone. When the tympanic is articulated the notch is converted into a foramen (stylomastoid foramen, Fig. 22, j)[35] for the exit of the seventh nerve. The fossa gives passage to the seventh nerve and also lodges the stapedius muscle. A groove may be traced from the stylomastoid foramen to the caudal border of the fossa for the tensor tympani muscle, where it passes into a canal (g). The groove and canal are parts of the canalis facialis or facial canal (aqueductus Fallopii) for the passage of the seventh nerve through the petrous bone.

The medial surface (Fig. 23, b) of the petrous portion shows near its middle a fossa, the internal auditory meatus (n). This is divided by a partition of bone into a dorsal and ventral part. The dorsal portion is the beginning of the facial canal (aqueductus Fallopii) by which the seventh nerve passes through the petrous bone to emerge at the stylomastoid foramen. The ventral portion shows at its bottom several small foramina for the auditory nerve.

Dorsocaudad of the internal auditory meatus is a deep fossa (o) for a small lobe, the so-called appendicular lobe, of the cerebellum. This may be called the appendicular fossa.

The dorsal surface is triangular and presents near its apex a foramen—the hiatus facialis (p), the opening of a canal which joins the canalis facialis and transmits the superficial petrosal branch of the nerve of the pterygoid canal (Vidian nerve). That part of the dorsal surface which lies caudad of the hiatus facialis is known as the tegmen tympani.

The base of the petrous is attached to the mastoid portion (Fig. 23, e).

(For an account of the structures within the petrous bone and the tympanic cavity, see the description of the internal and middle ear.)

The mastoid portion (Figs. 22 and 23, c) is attached by its base to the pyramidal petrous portion, with which it forms an angle of about 120 degrees. It appears in the lateral wall of the skull between the parietal bone and the occipital (Fig. 40, d). The lambdoidal ridge is continued on its outer surface to the caudal border of the external auditory meatus. Caudad of the stylomastoid foramen it forms a slight nipple-like eminence, the mastoid process (Fig. 22, l). Its inner face looks into the cranial cavity.

[36]

Parietal Bone. Os parietale

(Figs. 39, 40, and 43, 3).—The parietal bones form the larger part of the lateral and dorsal boundary of the cranial cavity. Each is a thin rectangular bone, compact and curved and with a deeply notched shelf of bone, the tentorium (Fig. 42, e, and Fig. 43, f), projecting inward from near the caudal margin.

The outer surface is smooth and convex. The highest part of the convexity, a little caudad of the middle of the bone, is known as the parietal tubercle or eminence (Fig. 39, d); it marks the point of beginning ossification. An obscure curved ridge (Fig. 39, e), running from the caudodorsal angle or a point craniad of it craniolaterad, indicates the boundary of the origin of the temporal muscle. Near the ventral border the surface is roughened and is covered in the natural state by a part of the squamous portion of the temporal bone.

The inner surface (Fig. 43, 3 and 3′) is smooth and marked by ridges and grooves for the convolutions of the cerebrum. Near the medial border is a ridge which, when the bone is articulated with that of the opposite side, forms a shallow groove for the superior sagittal sinus. Beginning near the middle of the ventral margin and passing dorsad is a groove for the middle meningeal artery. The tentorium (Fig. 43, f) arises from the inner surface near its caudal margin and projects mediad as a thin curved or notched shelf of bone which separates the cerebellar fossa (Fig. 43, I) of the cranium from the cerebral fossa (Fig. 43, II). When the parietals are articulated there is left between the tentoria a large foramen by means of which the two fossæ communicate. The foramen is bounded laterally and dorsally by the free margins of the tentoria, while the ventral end of each tentorium articulates with the alisphenoid, and its dorsal end with the opposite tentorium.

The medial border is straight and is united by suture to the opposite bone.

The cranial border is bevelled at the expense of the inner surface and articulates with the frontal. Just ventrad of the middle of the border projects a sharp spine which fits into a corresponding notch in the caudal border of the frontal.

[37]

The ventral border is concave, sharp, and bevelled at the expense of the outer surface, for articulation with the squamous portion of the temporal, except near the cranial end, where it articulates with the wing of the sphenoid.

The caudal border is thick and porous medially, but thin laterally, and bevelled at the expense of the inner surface for articulation with the interparietal and mastoid portion of the temporal.

Frontal Bone. Os frontale

(Figs. 39, 40, and 41, 5; Fig. 43, 8; Fig. 26).—The frontal bones meet one another in the median dorsal line so as to form the roof of the skull between the parietal and nasal bones. A part extends also ventrad, forming a large part of the medial wall of the orbit and a part of the temporal fossa.

The bone may be divided into two portions, a plate forming the cranial portion of the roof of the skull and a part of the roof of the nasal cavity, the frontal plate (Fig. 40, 5), and a part descending into the orbit, the orbital plate (Fig. 40, 5′).

The frontal plate (Fig. 40, 5) is a right-angled triangle with the hypothenuse lateral. Its dorsal surface is convex and smooth. The cranial two-thirds of its lateral border is separated from the orbital fossa by a ridge, the supraorbital arch or margin (Fig. 39, i; Fig. 40, o); the caudal third passes gradually into the temporal fossa. At its cranial angle is a triangular projection, the frontal spine or nasal spine (Fig. 26, a), which fits into a space between the nasal and maxillary bones.

Fig. 26.—Frontal Bone, Medial Surface.

a, frontal spine; b, transverse ridge; c, surface applied to the ethmoid; d, vertical plate of medial border.

The ventral surface is concave and smooth over its caudal one-half and helps to form the cranial part of the brain-case. It presents slight ridges and depressions for convolutions of the cerebrum. At its narrowed middle region the ventral surface is marked by a thick transverse ridge (Fig. 26, b). Caudally the ridge descends by a gentle slope to the level of the ventral[38] surface of the bone. The cranial end of the ridge is pierced by an oval foramen through which the frontal sinus (Fig. 43, m, m′), which lies within the ridge, communicates with the spaces in the ethmoid bone (nasal cavity). Craniad of the ridge the surface (Fig. 26, c) is rough and, together with the raised medial border of the bone and the orbital plate, encloses a rectangular space which in the natural state receives a portion of the labyrinth of the ethmoid. The ventral surface is marked at its medial edge by a thin longitudinal ridge which, when the bones are articulated, is continuous with one of the vertical lamellæ of the ethmoid.

The medial border forms a vertical plate (d), broadest craniad and roughened for articulation with its fellow of the opposite side except at its cranial end, where it articulates with the border of the nasal bone.

The caudal border is roughened, bevelled at the expense of the outer surface, and articulated with the parietal bone except at its ventral end, where it articulates with the alisphenoid.

The lateral border is smooth, and it is here that the orbital plate is joined to the frontal plate at right angles. Along its cranial two-thirds this union is marked by a sharp ridge, the supraorbital margin (Fig. 40, o) or arch. This ridge extends caudolaterad as a triangular projection, the zygomatic (or postorbital) process (Fig. 40, n), which is flattened on its cranioventral face near its extremity and forms part of the boundary of the orbital fossa. At its cranial end the lateral border articulates with the nasal and maxillary bones.

The orbital plate (Fig. 40, 5′) arises from the ventral surface of the lateral border of the frontal plate. It is directed ventrad, is smooth and concave on its outer surface, and forms the dorsal portion of the medial wall of the orbital fossa. Near its ventral border it bears the small ethmoidal foramen, for the artery of the same name.

On the caudal one-half of its inner surface (Fig. 26) it assists the caudal part of the dorsal plate in forming the brain-case. The cranial one-half of its inner surface is marked off from the remainder of the surface by a sharp irregular ridge[39] which is for articulation with the cribriform plate of the ethmoid. Craniad of this the surface is marked by ridges and looks into the nasal cavity.

The cranial margin is produced dorsally in the form of a blunt triangular spine. Mediad of this spine the bone articulates with the lachrymal bone.

The ventral border articulates by its cranial one-third with the orbital plate of the palatine, and by its caudal two-thirds with the body and wing of the presphenoid.

Maxillary Bone. Maxilla

(Figs. 27 and 28).—The maxillary bone forms the cranial and lateral portions of the roof of the mouth. The bones of opposite sides meet craniad, but diverge caudad to enclose the palatal plates of the palatine bones. Each consists of a thick prismatic ventral portion or body (a) and a thin flat plate, the frontal process (b), extending dorsad from the cranial part of the bone.

Fig. 27.—Maxillary Bone, Lateral Surface.

Fig. 28.—Maxillary Bone, Medial Surface.

a, body; b, frontal process; c, infraorbital foramen; d, elevation for root of canine tooth; e, canine tooth; f, first premolar; g, second premolar; h, third premolar; i, molar tooth; j, zygomatic process; k, beginning of lachrymal canal; l, ridge to which the ventral nasal concha is attached; m, nasal crest of palatine process.

The body (a) has the form of a triangular prism whose broader dorsal face looks into the nasal cavity and orbit, while the ventral face looks into the mouth, and the lateral face toward the cheek. From the junction of the dorsal and lateral surfaces at the cranial end the large flat curved frontal process (b) passes dorsad, while the teeth are implanted along the border, alveolar border or process, formed by the junction of the ventral and lateral surfaces.

[40]

The lateral surface is continuous with the lateral surface of the frontal process and shows at the base of the frontal process on its caudal border the large infraorbital foramen (Fig. 27, c), for the vessels and nerves of the same name. Near the medial end of the surface is a cylindrical elevation (d) for the root of the canine tooth (e).

The ventral surface is smooth and looks into the roof of the mouth.

On the dorsal surface caudal and cranial halves may be distinguished. The caudal one-half enters into the floor of the orbit. The lateral edge of this portion is divided into two laminæ, between which is received the end of the malar bone. Caudad this edge is prolonged into the short dorsally directed zygomatic process (j). The cranial half of the dorsal surface looks into the nasal cavity and is separated from the caudal half by a sharp vertical lamina of bone which runs caudomediad from the base of the nasal process. To the dorsal edge of this lamina are articulated the lachrymal bone and a part of the palatine. At the point where the lamina joins the base of the nasal process a foramen is seen leading into a canal, the nasolachrymal canal (k). Craniad of the lamina the surface is concave. Where it becomes continuous with the inner edge of the frontal process there is attached to it a thin bone, the ventral nasal concha (or maxilloturbinal), which is rolled into an irregular spiral. The nasolachrymal canal opens ventrad of its cranial end.

The cranial third of this part of the bone projects further mediad than does the rest of the medial border, forming thus the broad palatine process. This is rough on its medial edge for articulation with the premaxillary and the palatine process of the opposite bone. This medial edge rises also dorsally into a low ridge, the nasal crest (m), which is roughened for articulation with the vomer. The caudal two-thirds of the medial edge articulates with the palatine bone.

The cranial end of the bone articulates with the premaxilla.

The caudal end is smooth.

The frontal process (b) presents on its inner surface, which looks into the nasal cavity, certain transverse ridges which are[41] in relation with the ethmoid bone. Its outer surface is smooth. By its cranial border it articulates with the nasal bone dorsally and with the premaxillary bone ventrally.

Fig. 29.—Premaxillary Bone, Obliquely Craniolateral Aspect.

a, the three incisor teeth; b, palatal portion of the bone; c, nasal process.

Its dorsal end articulates medially with the nasal spine of the frontal bone, and caudally with the orbital plate of the same bone.

Premaxillary Bone. (Os incisivum BNA.) Premaxilla

(Fig. 29).—The premaxillary bones bear the incisor teeth and form the cranial portion of the roof of the mouth.

Each consists of an irregular, horizontal palatal portion (b) and of a perpendicular nasal process (c) which forms part of the lateral boundary of the nares and enters into the formation of the lateral wall of the nasal cavity.

The palatal portion has in its caudal border a deep notch for the foramen incisivum or anterior palatine canal, which lies between it and the maxillary and transmits blood-vessels and nerves. It articulates with the maxillary bone by this border.

The medial border is raised into a thin crest of bone which, besides forming the medial wall of the foramen incisivum or anterior palatine canal, articulates by its medial border with the bone of the opposite side, forming a sort of median trough (sulcus palatinus) which projects dorsad into the nasal cavity and receives the ventral border of the nasal septum. The caudal end of this border articulates laterad with the maxilla, dorsad with the vomer.

Its craniolateral border bears the incisor teeth (a).

The nasal process (c) presents three surfaces, all elongated and triangular; one, the medial surface, is smooth and concave and looks into the nasal cavity. Its dorsal border is rough for articulation with the nasal bone dorsad, and smooth ventrad where it aids in forming the nares.

The lateral surface is smooth.

The caudal surface is rough for articulation with the maxillary bone.

[42]

Nasal Bone. Os nasale

(Fig. 30).—The nasal bones fill the space between the nasal process of the premaxillary, the frontal process of the maxillary, and the nasal spine of the frontal bone (Fig. 39, 7). They thus form part of the dorsal wall of the nasal cavity near the middle line.

Fig. 30.—Nasal Bone, Dorsal View.

Each may be described as consisting of two elongated triangular lamellæ, one vertical, the other horizontal. The vertical lamella is curved slightly ventrad and has its apex directed craniad. It is applied by its medial surface against the vertical lamella of the opposite bone, the two thus forming a median vertical partition, the nasal crest (Fig. 43, 12), which extends ventrad into the nasal cavity and, by joining the dorsal edge of the lamina perpendicularis, helps to form the internasal septum.

The horizontal lamella is attached to the dorsal margin of the vertical lamella in such a way that its apex lies opposite the base of the vertical lamella. It helps to roof in the nasal cavity, and by its base forms a part of the dorsal boundary of the narial opening. By its lateral margin it articulates with the nasal spine of the frontal at its caudal end, with the frontal process of the maxillary at its middle, and with the nasal process of the premaxilla at its cranial end. The lateral angle of its base projects in a curved line which forms the dorsal part of the lateral boundary of the narial opening.

From the lateral border of the horizontal lamella a bony plate curves ventrad and mediad, enclosing a narrow fossa which receives a part of the ethmoid. This is the concha nasalis superior (nasoturbinal bone).

Ethmoid Bone. Os ethmoidale

(Figs. 31 and 32).—The ethmoid bone closes in the cranial cavity at its cranial end and extends forward into the nasal cavity, which it largely fills.

It consists of a median vertical portion, the lamina perpendicularis (Fig. 43, n; Fig. 42, p), forming a part of the nasal septum, of two lateral portions made of thin sheets of bone variously folded and united—the labyrinths (or ethmoturbinals), which fill the greater part of the nasal cavity; and of[43] a transverse perforated plate, the cribriform plate (lamina cribrosa), attached to the caudal end of the lamina perpendicularis and the labyrinths.

The lamina perpendicularis (Fig. 43, n; Fig. 42, p) is a flat four-sided bone. By its caudal margin it is continuous with the cribriform plate; by its ventral margin it is enclosed by the halves of the vomer; by its dorsal margin it unites with the crest formed by the vertical portion of the nasal bone craniad and with the vertical lamina of the medial margin of the frontal caudad, while its cranial margin is continued into the septal cartilage of the nose. Its lateral faces are smooth and free.

The lamina cribrosa or cribriform plate (Fig. 42, o) is elongated heart-shaped, with the apex of the heart ventrad. Its caudal face is concave and looks into the cranial cavity. It presents three irregular longitudinal rows of holes, one median and two lateral, for the passage of the olfactory fibres from the cranial cavity into the nasal cavity. Its cranial face is continuous along the medial line with the lamina perpendicularis, and at the sides with the labyrinths.

The notch in the heart is directed dorsad and receives the vertical lamina of the medial border of the frontal bone. The apex of the heart articulates with the cranial end of the dorsal surface of the presphenoid. Its lateral margins are articulated with the ethmoidal ridges on the medial surface of the frontal bone.

Fig. 31.—Ethmoid and Vomer, Side View.

Fig. 32.—Ethmoid and Vomer, Ventral View.

a, vomer; b, vertical cells of the labyrinth of the ethmoid; c, horizontal cell of the same; d, part of the ethmoid that forms the lamina papyracea; e, edge of cribriform plate.

The labyrinths (Figs. 31 and 32) are attached to the cranial face of the lamina cribrosa, one on each side of the lamina perpendicularis. Each is made of thin bony plates irregularly[44] folded so as to enclose spaces, the ethmoid cells. In each may be distinguished a cranial portion (b), in which the cells are nearly vertical, and a caudal portion (c), in which the cells are nearly horizontal.

The medial surfaces are separated by a space from the lamina perpendicularis. This space is broadest along the junction of the horizontal and vertical portions of the labyrinth. There are thus formed two passageways which correspond to the superior meati of human anatomy.

The lateral surfaces come into contact with the frontal process of the maxillary and the orbital plate of the frontal bone. On the lateral surface of each labyrinth there is a thin irregular lamina of bone lying in a dorsoventral longitudinal plane and closing in some of the ethmoid cells laterally (d). A small part of this lamina, situated near the caudoventral angle of the bone, appears in the orbital fossa on the external surface of the skull between the presphenoid, palatine, and frontal bones or between the lachrymal, palatine, and frontal bones. Sometimes in the entire skull two such pieces may be seen, one in each of these positions. This corresponds to the lamina papyracea of human anatomy.

The dorsocaudal angle of each bone is received into the space between the orbital plate of the frontal and the vertical lamina of the medial border of the frontal. Its ventrocaudal angle is received between the cranial extensions of the lateral walls of the presphenoid, while its ventral surface is overlaid caudally by the expanded portion of the vomer, to which it is attached at its caudolateral angles.

Vomer

(Figs. 31 and 32, a).—The vomer consists of two thin laminæ of bone which ensheath the ventral margin of the lamina perpendicularis (or the cartilaginous plate which continues ventrad from this margin) and unite ventrad of it; the two thus form a trough open dorsad.

Each becomes horizontal near its caudal end and at the same time expands. The expanded portion lies ventrad of the labyrinth of the ethmoid, closing in some of its cells: its lateral angles are united with the labyrinths.

At its caudal end the bone articulates with the body of the[45] presphenoid, and each half of it is produced caudad near the middle line into a triangular spine which lies ventrad of the body of the presphenoid. The horizontal portion of the bone helps to separate the olfactory and respiratory passages of the nasal chamber, while its vertical portion contributes to the formation of the nasal septum.

The ventral margin formed by the junction of the two halves of the bone is smooth and free caudad, but at its cranial end is broad and rough for articulation with the palatal processes of the maxillæ.

Palatine Bone. Os palatinum

(Fig. 33).—The palate bone or palatine bone consists of two portions, a horizontal or palatal portion (a) and a perpendicular or nasal portion (b), uniting at an angle of about forty-five degrees.

Fig. 33.—Palatine Bone, Dorsal View.

a, horizontal portion; b, perpendicular portion; c, maxillary spine; d, posterior nasal spine; e, sphenopalatine foramen; f, caudal opening of posterior palatine canal.

The horizontal portions (a) of the two bones are received between the maxillary bones and form the caudal and medial part of the roof of the mouth. Each is irregularly quadrilateral in form, with the caudolateral angle produced caudad into a long process which is continuous with the perpendicular portion of the bone. The lateral margin of the horizontal portion articulates over its cranial half with the maxillary bone. At about its middle a short thick maxillary spine (c) projects caudolaterad. The remainder of the lateral margin is directly continuous with the perpendicular plate of the bone. The medial margin is rough for articulation with the corresponding margin of the opposite palatine; the caudal angle of this margin projects caudad as the short posterior nasal spine (d). The caudal margin forms a free edge which bounds the choanæ; it passes laterally into the perpendicular portion.

The ventral surface (Fig. 41, 8) looks into the mouth. Near the middle of its craniolateral margin are two or more small foramina (Fig. 41, q) which form the cranial termination of the posterior palatine canal. The dorsal surface is smooth and looks into the nasal cavity.

[46]

The perpendicular or nasal portion (Fig. 33, b) of the palatine is thin and irregularly quadrilateral in form. It is attached by its cranial two-thirds to the dorsal surface of the horizontal portion. The outer surface is concave and looks into the orbital fossa. The inner surface is convex and looks into the nasal cavity.

The perpendicular portion is marked by two foramina just craniad of the middle. The larger dorsal oval foramen is the sphenopalatine foramen (e). The smaller ventral foramen is the caudal opening of the posterior palatine canal (f). From this opening the canal passes craniomediad, lying in the substance of the palatine bone; it opens on the ventral surface of the horizontal portion at the small openings previously described (Fig. 41, q).

By its cranial margin it articulates with the lachrymal bone. By its dorsal margin it articulates craniad with the orbital plate of the frontal: with the lamina papyracea at its middle, and with the body of the presphenoid caudad. The caudal half of the dorsal margin is partially divided into two lamellæ with a rough surface between them: this rough surface lies against the ventral surface of the presphenoid. The caudal margin articulates with the pterygoid portion of the sphenoid.

Lachrymal Bone. Os lachrymale

(Fig. 34; Fig. 39, 10).—The lachrymal bone is a thin pentagonal scale of bone filling the interval between the horizontal plate of the palatine, the maxillary, and the orbital plate of the frontal. Its outer surface looks into the orbit, its inner surface into the nasal cavity.

Fig. 34.—Lachrymal Bone of Left Side, External Surface.

Fig. 35.—Malar Bone of Right Side, Lateral Surface.

Fig. 34.—a, notch forming the beginning of the lachrymal canal.

Fig. 35.—a, ridge for origin of the masseter muscle; b, frontal process; c, zygomatic process.

[47]

Near the middle of its cranial border it is notched obliquely by a foramen (a), the beginning of the nasolachrymal canal.

Malar Bone. Jugal Bone. Os zygomaticum

(Fig. 35).—The malar or zygomatic bone is a flat curved plate of bone which forms the lateral wall of the orbit and together with the zygomatic process of the temporal forms the zygomatic arch. Its outer surface is smooth and marked by a longitudinal ridge (a) for attachment of the masseter muscle.

At its caudal end the bone is continued into two processes: one, the frontal process or orbital process (b), is a triangular spine of bone directed caudomediad; when the bones are articulated it lies opposite the zygomatic process of the frontal to which it is joined by a ligament (orbital ligament). The other, zygomatic process (c) of the malar bone, extends ventrocaudad and articulates with a similar process from the temporal to form the zygomatic arch above mentioned.

Its inner surface is smooth and looks into the orbit, except that of the zygomatic process, which looks into the temporal fossa.

Its cranial border is roughened at the expense of both surfaces and articulates with the maxillary bone. Its other borders are smooth except the dorsal border of the zygomatic process, which is roughened for attachment to the zygomatic process of the temporal.

The Mandible. Mandibula

(Figs. 36 and 37).—The mandible (or inferior maxillary bone) is composed of two halves which come together at the cranial end and form the lower jaw. At its caudal end each half articulates with the temporal bone at the mandibular fossa, and at its cranial end it joins the opposite bone, the suture being known as the symphysis of the jaw (symphysis menti) (Fig. 37, a).

Each half consists of a horizontal portion, the body (b), bearing teeth on one of its borders (the alveolar border), and of a vertical portion, the ramus (c).

The body (b) has the form of a flattened cylinder and has two surfaces and two borders. The lateral surface (Fig. 36) is smooth and presents near its cranial end a foramen (or sometimes two), the mental foramen (d), forming the cranial[48] termination of the mandibular canal. At its caudal end is a deep fossa continuing on to the ramus, the coronoid fossa, or masseteric fossa (e).

Fig. 36.—Mandible, Lateral Surface.

Fig. 37.—Mandible, Medial Surface.

a, symphysis; b, body; c, ramus; d, mental foramina; e, coronoid fossa; f, mandibular foramen; g, angular process; h, coronoid process; i, condyloid process; 1, 2, 3, the three incisor teeth; 4, the canine tooth; 5, 6, the premolars; 7, the molar tooth.

The medial surface (Fig. 37) is smooth and has near its caudal end a foramen, the mandibular foramen (f), which communicates with the mandibular canal leading lengthwise through the bone to the mental foramen. The cranial end is roughened for attachment to the bone of the opposite side.

The ventral border is smooth and rounded; it ends caudally in a blunt point, the angular process (g). The dorsal (alveolar) border is slightly curved and bears the sockets (alveoli) for the teeth. It is continuous with the cranial margin of the coronoid process.

The ramus is divided into two portions, the coronoid process (h) and the condyloid process (i). The coronoid process (h) extends dorsocaudad as a thin plate of bone with smooth surfaces and borders. Its outer surface is partly occupied by the coronoid fossa (e). The condyloid process (i) has the form of[49] a semicylindrical transverse piece of bone attached to the caudal margin of the coronoid process. It articulates with the mandibular fossa of the temporal bone.

Hyoid Bone. Os hyoideum

(Fig. 38 and Fig. 104).—The hyoid bone forms the support for the tongue and gives origin to muscles passing to the tongue and larynx. It also supports the thyroid cartilage (Fig. 104, 1).

Fig. 38.—Hyoid Bone, Dorsal View.

a, body; b, c, d, e, cranial cornu; f, caudal cornu; b, ceratohyal; c, epihyal; d, stylohyal; e, tympanohyal; f, thyrohyal.

It consists of a transverse bony bar, the body (Fig. 38, a) and of two cornua or horns attached to each end of the body.

The cranial cornu (lesser cornu of human anatomy) is the longer (Fig. 38, b-e). Each arises from the cranial face of the body at its lateral end, curves laterad, and then caudodorsad. It consists of four bony pieces movably united by cartilage.

The terminal piece is the tympanohyal (e); it is imbedded in the tympanic bulla just ventrad of the stylomastoid foramen. It is not therefore seen attached to the cornu after the latter has been separated from the skull. The other pieces become successively shorter toward the body, and are called stylohyal (d), epihyal (c), and ceratohyal (b).

The caudal cornua (f) (greater cornua of human anatomy) arise from the ends of the body. Each consists of a single piece of bone, the thyrohyal (f), which passes caudolaterad; its free end is united to a process of the thyroid cartilage (Fig. 104, 1).

The Skull as a Whole.

—In the following description of the skull as a whole the mandible, hyoid, and ear-bones are not included.

The skull forms a bony box which contains the brain and is produced craniad into the facial portion which encloses the nasal cavity and forms the framework of the face.

In dorsal view (Fig. 39) the skull presents a smooth convex surface, broadest caudad, with the two zygomatic arches[50] (g) curving out some distance laterally. The following bones are visible in dorsal view: the occipital (1), interparietal (2), parietals (3), temporals (4), frontals (5), malar or zygomatic bones (6), nasals (7), maxillaries (8), premaxillaries (9), and lachrymals (10).

Fig. 39.—Skull, Dorsal Surface.

1, occipital bone; 2, interparietal bone; 3, parietal bones; 4, temporal; 5, frontal; 6, malar; 7, nasal; 8, maxillary; 9, premaxillary; 10, lachrymal, a, lambdoidal ridge; b, external occipital tubercle; c, sagittal crest; d, parietal eminence; e, line which forms the dorsal boundary of the temporal fossa; f, zygomatic process of the frontal; g, zygomatic arch; h, frontal process of the malar; i, supraorbital arch; j, nares; k, foramen incisivum or anterior palatine foramen; l, sphenopalatine foramen; m, zygomatic process of the temporal; n, infraorbital foramen; o, opening of lachrymal duct.

The caudal boundary of the dorsal surface is marked by the prominent lambdoidal ridge (a) which passes from the middle cranioventrad along each side to the root of the zygomatic arch: it is borne by the occipital and temporal bones. From the middle of the lambdoidal ridge a second ridge, the sagittal[51] crest (c), passes craniad in the middle line across the interparietal bone: it varies greatly in extent, reaching in a very old and muscular cat to the cranial border of the parietals, while in kittens it does not exist. The most prominent portions of the skull in this region, just craniad of the middle of the parietal bones, are known as the parietal tubercles or eminences (d). A faint curved line (e) runs from the cranial end of the sagittal crest craniolaterad to the base of the zygomatic process of the frontal: it marks the dorsal boundary of the origin of the temporal muscle, and may therefore be considered the dorsal boundary of the temporal fossa. This fossa extends from its dorsal boundary as far laterad and caudad as the lambdoidal ridge (a), and as far craniad as a line connecting the tip of the zygomatic process of the frontal (f) with the frontal process of the malar (h). The temporal muscle takes origin from its surface.

The middle portion of the dorsal surface is formed by the frontals (5). Each frontal presents laterally a prominent zygomatic process (f), extending ventrolaterad toward a corresponding (frontal) process (h) of the malar bone. These two processes mark the boundary between the orbital fossa (craniad) and the temporal fossa (caudad). Craniad of the zygomatic process of the frontal a sharp margin separates the dorsal surface of the skull from the wall of the orbital fossa: this is the supraorbital arch or margin (i).

The cranial portion of the dorsal surface is formed by the maxillary (8), nasal (7), and premaxillary bones (9). Just craniad of the nasals, bounded ventrad and craniad by the premaxillaries, appears the large opening of the nares (j), leading into the nasal cavity.

The zygomatic arch (g) is formed by the zygomatic process of the temporal (m) and the malar or zygomatic bone (6). Each presents near its middle a prominent dorsocaudally directed process, the frontal process (h) of the malar bone. The zygomatic arch forms the lateral boundary of the temporal and orbital fossæ, which are separated by a line connecting the frontal process of the malar (h) and the zygomatic process of the frontal (f).

[52]

A portion of the floor of the orbit and the opening of the lachrymal canal (o) may also be seen in dorsal view; they are described in connection with the lateral surface.

The caudal surface of the skull is formed largely by the occipital bone (Fig. 17), surrounding the foramen magnum (Fig. 17, d). At the sides of the foramen magnum are the two prominent curved occipital condyles (e) for articulation with the atlas. Craniolaterad of the condyles, separated from them by a deep notch, are the jugular processes (f) of the occipital, closely applied to the caudal ends of the tympanic bullæ.

Dorsad of the foramen magnum are faint indications of a median ridge running dorsad, the external occipital crest (Fig. 17, i); this rises at its junction with the lambdoidal ridge to form the prominent external occipital tubercle (Fig. 39, b). The dorsal and dorsolateral boundaries of the posterior surface are formed by the lambdoidal ridge (Fig. 17, h; Fig. 39, a).

The lateral surface of the skull (Fig. 40) is much more complicated than the dorsal and posterior surfaces. Caudally the occipital condyles (a) and external occipital crest (b) are visible; dorsocaudad the sagittal crest (c).

Extending from the caudal end of the sagittal crest the lambdoidal ridge (d) is seen passing ventrocraniad to the tympanic bulla, thence craniad to the root of the zygomatic arch. In the ventral part of the caudal region the tympanic bulla (e) is visible with the jugular process (f) of the occipital pressed close against its caudal end. Just craniad of the jugular process the mastoid process (g) of the temporal rests against the side of the bulla. Beneath the cranial edge of this process is the opening of the stylomastoid foramen (h) for the seventh nerve, while just ventrad of the foramen is the small pit (i) in the tympanic bulla for the reception of the tympanohyal bone. Craniad of the stylomastoid foramen is the large opening of the external auditory meatus (j), leading into the middle ear.

Immediately dorsocraniad of the external auditory meatus the zygomatic arch begins as the zygomatic process (k) of the temporal bone. On the cranial surface of the base of this process is the deep mandibular fossa (l) for the condyle of the[53] mandible. This fossa is bounded caudally by the prominent postmandibular process (m).

Fig. 40.—Skull, Side View.

1, occipital bone; 2, interparietal; 3, parietal; 4, temporal; 5, 5′, frontal; 6, malar; 7, sphenoid; 8, palatine; 9, presphenoid; 10, maxillary; 11, nasal; 12, premaxillary; 13, incisor teeth; 14, canine; 15, 16, 17, premolars; 18, molar. a, occipital condyle; b, external occipital crest; c, sagittal crest; d, lambdoidal ridge; e, tympanic bulla; f, jugular process; g, mastoid process; h, stylo-mastoid foramen; i, pit for tympanohyal bone; j, external auditory meatus; k, zygomatic process of temporal bone; l, mandibular fossa; m, postmandibular process; n, zygomatic process of the frontal; o, supraorbital margin; p, external pterygoid fossa; q, sphenopalatine foramen; r, orbital fissure; s, internal pterygoid fossa; t, hamulus; u, foramen ovale; v, foramen rotundum; w, optic foramen; x, opening of lachrymal canal; y, infraorbital foramen.

All that portion of the lateral surface of the skull which lies craniodorsad of the lambdoidal ridge may be divided (excluding the zygomatic arch) into three main parts, the temporal fossa, the orbital fossa, and the face. The boundaries of the temporal fossa have been given. The orbital fossa is bounded externally by a prominent semicircular ridge formed chiefly by the zygomatic arch, the zygomatic process of the frontal (n), and the supraorbital arch (o) of the frontal, which may be traced to the cranial root of the zygomatic arch. The orbital fossa may be considered to end caudally and ventrally at the level of the optic foramen (w); ventrad of it are certain smaller fossæ. Immediately ventrad is the long external pterygoid fossa (p), from which arises part of the external pterygoid muscle. This fossa begins at the sphenopalatine foramen (q) and extends caudad to the orbital fissure (r); it is separated by a ridge from the orbital fossa. Caudoventrad of the external[54] pterygoid fossa and separated from it by a sharp ridge is the small narrow internal pterygoid fossa (s), which extends ventrad without interruption on to the surface of the hamulus (t) and caudad to within two or three millimeters of the tympanic bulla. From it the internal pterygoid muscle takes origin. The hamulus (t) projects caudoventrad in this region, forming a prominent feature in a lateral view.

Four foramina leading into the cranial cavity are visible in a lateral view of the skull, craniad of the tympanic bulla. The one nearest the bulla is the foramen ovale (u) for the third division of the fifth nerve; next craniad of this is the foramen rotundum (v) for the second division of the fifth nerve. These two foramina pierce the alisphenoid: just craniad of them, between the alisphenoid and the orbitosphenoid, is the large orbital fissure (r) (foramen lacerum anterius), which transmits the third, fourth, and sixth cranial nerves and the first division of the fifth. Dorsocraniad of the orbital fissure is the optic foramen (w), for the optic nerve.

Ventrad of the cranial portion of the orbit is the large sphenopalatine foramen (q), for the nerves and arteries of the same name. Just craniad of this is the small caudal opening of the posterior palatine canal, which passes through the substance of the palatine bone and opens on its ventral surface near its cranial margin. Just dorsad of the cranial root of the zygomatic arch is the opening of the lachrymal canal (x), while the root of the arch is pierced by the large infraorbital foramen (y), which transmits the infraorbital nerves and artery from the orbit.

The teeth (13-18), implanted along the alveolar border of the maxillary and premaxillary, form a prominent feature in a lateral view: they are described in the account of the alimentary canal.

The ventral surface of the skull (Fig. 41) is very complex. It is separated by the orbits into a caudal and a cranial portion, united by a narrow median trough-like part. Laterad of this trough-like part are visible parts of the orbit and the zygomatic arches, which do not properly belong to the ventral surface and have already been described.

[55]

Fig. 41.—Skull, Ventral View.

1, occipital bone; 2, temporal; 3, sphenoid; 4, presphenoid; 5, frontal; 6, malar; 7, vomer; 8, palatine; 9, maxillary; 10, premaxillary. a, foramen magnum; b, occipital condyles; c, jugular process; d, tympanic bulla; e, mastoid process; f, stylomastoid foramen; g, external auditory meatus; h, jugular foramen; i, styliform process; j, groove for Eustachian tube; k, foramen ovale; l, foramen rotundum; m, pterygoid process of sphenoid; n, perpendicular plate of palatine; o, choanæ or posterior nares; p, zygomatic arch; q, cranial end of posterior palatine canal; r, palatine grooves; s, foramina incisiva or anterior palatine foramina; t, opening of pterygoid canal.

Caudally there appear in the ventral view the foramen magnum (a), occipital condyles (b), and jugular processes (c). In front of the jugular processes the two tympanic bullæ (d) form prominent features, with the mastoid process (e), the stylomastoid foramen (f), and the external auditory meatus (g) on their lateral surfaces. All these structures have been described. The tympanic bullæ (d) are placed with long axes directed craniomediad, so that they converge toward their cranial ends. At the caudomedial angle of each bulla is the[56] large jugular foramen (h), for the ninth, tenth, and eleventh nerves. Opening into the mediocaudal margin of the jugular foramen is the smaller hypoglossal foramen, for the twelfth nerve.

The craniomedial end of the tympanic bulla projects craniad as the styliform process (i). Just laterad of this process is the opening (j) into the tympanic bulla by which the tuba auditiva or Eustachian tube passes into the middle ear. A faint groove for the tube passes craniomediad from this opening, on the surface of the sphenoid. Craniolaterad of the opening for the tuba auditiva is the foramen ovale (k); craniad of this the foramen rotundum (l) is faintly indicated. On the surface of the sphenoid just craniad of the styliform process of the bulla tympani is the minute opening of the pterygoid canal (t). The orbital fissure and optic foramen are not seen in the ventral view.

The middle region of the ventral surface is narrow: it is formed by a trough-like fossa which is bounded laterally by the pterygoid processes (m) of the sphenoid and the perpendicular plates of the palatines (n). Ventrad of this lies, in the natural condition, the soft palate, converting the fossa into the nasal portion of the pharynx or nasopharynx. Craniad this fossa is bounded by the free caudal edges of the palatines; beneath which the fossa communicates with the nasal cavity by the two choanæ (o). Laterad of this median fossa are visible in the ventral view parts of the temporal and orbital fossæ, bounded laterally by the zygomatic arches (p).

The cranial part of the ventral surface is a somewhat triangular plane area formed by the palatal portions of the palatines (8), maxillaries (9), and premaxillaries (10), which together constitute the hard palate (palatum durum). Laterad and craniad this area is bounded by the alveolar borders of the maxillaries and premaxillaries bearing the teeth. The hard palate is marked near the cranial border of the palatine bones with two or more foramina which form the cranial termination of the posterior palatine canal (q). Two faint grooves pass from these foramina a short distance craniad, gradually converging: these are known as the palatine grooves (r)[57] (sulci palatini). Near the cranial end of the hard palate are two large openings close together near the middle line: these are the foramina incisiva (or anterior palatine foramina) (s).

Fig. 42.—Skull, with Dorsal Surface Removed, showing the Cranial and Nasal Cavities.

a, foramen magnum; b, caudal end of hypoglossal canal; c, jugular foramen; d, internal auditory meatus; e, tentorium, forming the cranial boundary of the cerebellar fossa; f, dorsum sellæ; g, sella turcica; h, anterior clinoid processes; i, foramen ovale; j, foramen rotundum; k, orbital fissure; l, optic foramen; m, chiasmatic groove; n, presphenoid bone; o, cribriform plate; p, lamina perpendicularis of ethmoid; q, labyrinths of ethmoid; r, nares; s, foramina incisiva or anterior palatine foramina; t, infraorbital foramen; u, opening of the lachrymal canal; v, caudal opening of posterior palatine canal; w, sphenopalatine foramen; x, frontal process of the malar; y, zygomatic process of the temporal; z, appendicular fossa, in the petrous bone.

Cavities of the Skull

(Figs. 42 and 43).—The bones of the cranial portion of the skull enclose the cranial cavity for the brain; the facial bones enclose the nasal cavity, for the olfactory organ.

The cranial cavity is divisible into three principal fossæ: the cerebellar fossa (Fig. 43, I) caudad, for the cerebellum;[58] the cerebral fossa (II) in the middle, for the cerebrum; the small olfactory fossa (III) at the cranial end for the olfactory bulb of the brain.

The cerebellar fossa (I) is bounded caudally by the occipital bone enclosing the foramen magnum (Fig. 42, a). Its ventral surface is formed by the basilar portion of the occipital and the petrous portions of the temporals; its lateral surface by the mastoid portions of the temporals and parts of the parietals and occipital. Its roof is formed by the parietals and interparietal. Craniad the cerebellar fossa is partly separated from the cerebral fossa by the tentorium (Fig. 42, e; Fig. 43, f) formed by the two parietals: this encloses a quadrangular opening by which the two fossæ communicate. The caudal, dorsal, and lateral walls of the cerebellar fossa are deeply marked by fossæ for the lobes of the cerebellum; the small appendicular fossa (Fig. 43, e), forming a deep indentation in the petrous bone near its dorsocaudal end, is particularly noticeable.

The following openings are found in the walls of the cerebellar fossa. Caudad is the large foramen magnum (Fig. 42, a) by which the brain-cavity communicates with the vertebral canal. Near the caudal margin of the foramen magnum, on its lateral side, just mediad of the dorsal end of the occipital condyle, is the caudal opening of the condyloid canal (Fig. 43, a) which passes craniad through the substance of the occipital bone to open just caudad of the petrous: it transmits a vein. The condyloid canal varies greatly in size in different specimens. A few millimeters craniad of the edge of the foramen magnum on the floor of the fossa is the small opening of the hypoglossal canal (Figs. 42 and 43, b), for the twelfth nerve. Just craniad of this, at the caudomedial border of the petrous, is the large jugular foramen (c). On the petrous itself, near the middle, is the internal, auditory meatus (d) divided into the dorsal facial canal for the seventh nerve, and a ventral passage for the eighth nerve. At the cranial end of the cerebellar fossa is the large opening bounded by the free edges of the tentorium.

The cerebral fossa forms much the largest part of the cranial cavity. It is bounded by the parietals (Fig. 43, 3′),[59] squamous portions of the temporals (4), frontals (8), the sphenoid (5), and presphenoid (6). A slight rounded ridge on its lateral wall at about the position of the suture between the frontals and parietals separates a smaller cranial portion sometimes called the anterior fossa, from a larger caudal portion sometimes known as the middle fossa of the cranial cavity. The walls of the cerebral cavity are marked with numerous ridges and shallow furrows for the cerebral convolutions.

The floor of the cerebral cavity is bounded caudad by the prominent dorsum sellæ (Fig. 42, f; Fig. 43, g), just craniad of which is the rounded depression known as the sella turcica (Fig. 42, g; Fig. 43, h), for lodgment of the hypophysis. A number of foramina pierce the floor of the cavity in this region. Just ventrad of the cranial tip of the petrous portion of the temporal is the small foramen lacerum (medius). Craniad and laterad of this is a row of four foramina: the caudal one is the foramen ovale (Fig. 42, i); then come in order the foramen rotundum (j), the orbital fissure (k), and the optic foramen (Fig. 42, l; Fig. 43, k). The two optic foramina are connected by the shallow transverse chiasmatic groove (Fig. 42, m), for the optic chiasma. Another small foramen continues caudad from a groove on the floor of the orbital fissure; this opens on the ventral surface of the sphenoid, between the wing and the body of the bone. The groove and foramen constitute the pterygoid canal, which transmits a nerve,—the nerve of the pterygoid canal, or Vidian nerve.

The cranial cavity narrows at its cranial end to form the small olfactory fossa (Fig. 43, III) which lodges the olfactory bulbs. This is bounded by the frontals and the lamina cribrosa (Fig. 42, o) of the ethmoid; caudad it opens directly into the cerebral fossa. Numerous openings through the lamina cribrosa for the olfactory fibres connect the olfactory fossa with the nasal cavity. The roof of the fossa is marked by a prominent median crest from the united edges of the frontals.

The nasal cavity is almost completely filled by the ethmoid and vomer and the conchæ nasales. Its roof is formed by the nasal bones and portions of the frontals; its sides by the frontals, lachrymals, maxillaries, premaxillaries, and palatine[60] bones; its floor by the horizontal plates of the palatines, maxillaries, and premaxillaries.

Fig. 43.—Skull, Median Longitudinal Section, showing the Cavities.

I, cerebellar fossa; II, cerebral fossa; III, olfactory fossa. 1, occipital bone; 2, interparietal; 3, 3′, parietal; 4, temporal (4, squamous portion; 4′, petrous portion; 4″, tympanic portion); 5, sphenoid; 6, presphenoid; 7, palatine; 8, frontal; 9, maxillary; 10, premaxillary; 11, ethmoid; 12, nasal; 13, incisor teeth; 14, canine; 15, 16, 17, premolars; 18, molar. a, condyloid canal; b, hypoglossal canal; c, jugular foramen; d, internal auditory meatus; e, appendicular fossa; f, tentorium; g, dorsum sellæ; h, sella turcica; i, hamular process; j, pterygoid process of sphenoid; k, optic foramen; l, presphenoid sinus; m, m′, frontal sinus; n, lamina perpendicularis of the ethmoid (broken at cranial edge).

The nasal cavity opens craniad by the large nares (Fig. 39, j; Fig. 42, r), which are bounded by the premaxillary and nasal bones. In the natural condition this opening is divided by a median cartilage which is continuous with the lamina perpendicularis (Fig. 43, n) of the ethmoid, thus forming a partition which divides the nasal cavity into two separate halves. From the floor of the cranial part of the cavity rises a ridge formed of the nasal crests of the maxillaries and premaxillaries, and the cranial portion of the vomer. Farther caudad the vomer spreads out in a horizontal plane and separates from the floor of the cavity, so that the nasal cavity is thereby divided by a horizontal partition into dorsal and ventral portions. The ventral portion is small, forming the inferior meatus of the nose; it ends caudally at the choanæ (posterior nares, Fig. 41, o) which lead into the nasopharynx. That portion of the nasal cavity lying dorsad of the vomer is almost completely[61] filled by the ethmoid and the conchæ nasales, superior and inferior. It is bounded caudally by the lamina cribrosa of the ethmoid (Fig. 42, o). The nasal cavity communicates with the cranial cavity by the foramina for the olfactory fibres in the lamina cribrosa; with the nasopharynx by the choanæ; with the exterior of the body by the nares; with the mouth-cavity by the foramina incisiva or anterior palatine foramina (Fig. 42, s); with the orbit by the sphenopalatine foramen and the nasolachrymal canal. It communicates directly also with the frontal sinuses (Fig. 43, m, m′), the sphenoidal sinuses (Fig. 43, l), and with the cells of the labyrinths of the ethmoid.

JOINTS AND LIGAMENTS OF THE SKULL.

Sutures of the Skull.—The bones of the skull join each other by means of immovable articulations known as sutures. These sutures are designated by combining the names of the bones between which they are situated: as, sphenofrontal suture (sutura sphenofrontalis), between the sphenoid and frontal; nasomaxillary suture (sutura nasomaxillaris), between the nasal and maxillary bones. When a suture joins the two corresponding bones of opposite sides the prefix inter is used, as the intermaxillary suture (sutura intermaxillaris) between the maxillaries. The sutures bounding the parietals have, however, received special names not derived in this manner. The suture caudad of the parietals, separating them from the occipital and interparietal, is known as the lambdoidal suture; that between the two parietals is the sagittal suture; that separating the parietals and squamous portions of the temporals is the squamous suture; that between the parietals and frontals is the coronal suture. The suture separating the two frontals also is known as the frontal suture, in place of interfrontal.

Articulations of the Mandible.—In man the two halves of the mandible are united craniad, so as to form a single bone. In the cat the two halves are separate, but articulate closely at the symphysis menti by a thin interarticular cartilage.

The articulation of the mandible at the mandibular fossa of the temporal is covered with a close articular capsule. The[62] mandibular fossa is lined with cartilage. A slender ligament passes from the angular process of the mandible caudad to the external auditory meatus, being attached to the latter about 8 millimeters from its medial end. This is the stylomandibular ligament.

V. BONES OF THE THORACIC EXTREMITIES.

Scapula

(Figs. 44 and 45).—The scapula may be described as a flat triangular bone with one angle rounded. It lies beneath the muscles on the lateral face of the thorax near its cranial end. From its lateral surface there projects a flat ridge (Fig. 44, g), the spine of the scapula. The ventral end of the ridge is free as a curved process, the acromion process (Fig. 44, j).

Fig. 44.—Scapula, Lateral Surface.

Fig. 45.—Scapula, Medial Surface.

I, fossa supraspinata; II, fossa infraspinata; III, fossa subscapularis; IV, fossa for teres major. a, vertebral border; b, coracoid border; c, glenoid border; d, glenoid angle and fossa; e, coracovertebral angle; f, glenovertebral angle; g, spine; h, tuberosity of the spine; i, metacromion; j, acromion; k, supraglenoidal tubercle; l, incisura scapulæ; m, coracoid process; n, groove indicating portion of spine; o, o′, ridges for attachment of muscle-fibres.

The ventral angle of the scapula (d), the glenoid angle[63] (lateral angle of human anatomy), is much heavier than the others and bears a concave, pear-shaped articular facet, the glenoid fossa, for articulation with the humerus. The border with which this angle is more nearly continuous may be called the glenoid border (c) (axillary border of human anatomy).

Near the narrower cranial end of the glenoid fossa is a small curved projection of the bone, the coracoid process (Fig. 45, m). The border upon which it lies is the coracoid border (b) (superior border of human anatomy). The third border is turned toward the vertebral column and is the vertebral border (a).

The angle between the glenoid and vertebral borders is the glenovertebral angle (f) (inferior angle of human anatomy), and that between the coracoid and vertebral borders the coracovertebral angle (e) (medial angle of human anatomy).

The medial or costal surface (Fig. 45) is smooth and nearly flat. A shallow furrow (n) marks the position of the spine of the scapula. Between the furrow and the coracoid border are two oblique parallel ridges (o and o′) for the insertion of muscle-fibres. Near the glenoid border is a well-marked ridge separating the subscapular fossa (III), comprising the greater part of the medial surface of the scapula, from the fossa in which the teres major muscle has origin (IV). The surface presents several nutrient foramina usually directed toward the glenoid angle.

The lateral surface (dorsal surface of human anatomy) (Fig. 44) is divided by the spine (g) into two portions. The portion of the scapula craniad of the spine and the cranial surface of the spine bound the supraspinous fossa (fossa supraspinata) (I), while the surface caudad of the spine and the caudal portion of the spine bound the infraspinous fossa (fossa infraspinata) (II).

The spine (g) begins as a triangular elevated area in the middle of the vertebral margin and runs toward the glenoid angle. It rises gradually for about two-fifths of its length and then the margin becomes broader and the spine remains of the same height to its glenoid end. There is a rough thickening, the tuberosity (h) of the spine, situated on its free border about[64] midway between the tip of the acromion and the vertebral end of the spine. The spine is inclined toward the glenoid margin so as to form an angle of about 60 degrees with the caudal half of the lateral surface.

At the base of the acromion process (j) the margin of the spine presents a flat triangular projection, the metacromion (i), directed toward the glenoid border.

The acromion (j) continues in the direction of the spine. It is thicker than the spine, smooth and rounded on both its surfaces and both its borders, and its apex is connected by fibrous tissue to the clavicle.

The coracoid border (b) presents a slight rounded notch, the incisura scapulæ or suprascapular notch (l), just dorsad of the glenoid angle, and at its ventral end bears the coracoid process (m) which is directed ventromediad.

The glenoid angle (d) is the only one requiring special mention. Between the root of the coracoid process and the glenoid cavity it presents a tubercle, the supraglenoidal or bicipital tubercle (k), for the tendon of origin of the biceps muscle. The glenoid angle is separated by a contracted neck from the rest of the bone. Between this angle and the inner margin of the acromion there is left a deep notch, the great scapular notch.

Fig. 46.—Clavicle.

a, medial end; b, lateral end.

Clavicle. Clavicula

(Fig. 46).—The clavicle in the cat is greatly reduced. It is a slender curved rod of bone imbedded in the muscles of the shoulder and connected by fibrous tissue to the apex of the acromion process. The lateral end (b) is slightly enlarged.

Humerus

(Figs. 47 and 48).—The humerus forms the support of the upper arm and articulates by its proximal end with the scapula at the glenoid cavity, and by its distal end with the radius and ulna, the bones of the lower arm. It is a nearly cylindrical bone with enlarged ends, and is so curved that its dorsal and ventral borders are hooked at the opposite ends so that it has the form of an Italic f.

The proximal end of the bone bears on its dorsomedial portion a thickening, the head of the humerus (a), which bears[65] a smooth ovoid articular facet by which the bone articulates with the glenoid cavity of the scapula. The head is not separated from the body by a distinct anatomical neck as in the human humerus.

Fig. 47.—Humerus, Ventral Side.

Fig. 48.—Humerus, Medial Side.

a, head; b, greater tuberosity; c, lesser tuberosity; d, bicipital groove; e, pectoral ridge; f, deltoid ridge; g, rough area for insertion of latissimus dorsi and teres major; h, nutrient foramen; i, capitulum; j, trochlea; l, coronoid fossa; m, radial fossa; n, medial epicondyle; o, lateral epicondyle; q, supracondyloid foramen.

Along the lateral border of the proximal end of the shaft is a high rough ridge semicircular in side view, the great tuberosity (b). It gives attachment to muscles and is marked on its dorsal border by a deep depression for the tendon of the infraspinatus muscle. On the medial margin of the proximal end closely associated with the head is a smaller elevation, the lesser tuberosity (c), also for muscular attachment.

Between the greater and lesser tuberosities on the ventral[66] surface is seen a broad groove, the sulcus intertubercularis or bicipital groove (d), which passes distad onto the surface of the shaft. In the natural state it is converted into a canal by overlying tendons and lodges the tendon of the biceps muscle.

The shaft is nearly cylindrical at its middle, but its dorsoventral diameter is slightly greater than its mediolateral diameter. Its proximal end is flattened mediolaterad, while its distal end is flattened dorsoventrad.

From the ventral margin of the greater tuberosity a ridge, the pectoral ridge (e), is continued onto the surface of the shaft, and from the dorsal margin another ridge, the deltoid ridge (f), passes distad and ventrad so as to meet the pectoral ridge near the middle of the ventral surface of the bone. On the medial margin of the bone near the junction of the first and second fourths is a roughened area (g) for the attachment of the tendons of the latissimus dorsi and teres major muscles, and on the same surface near the junction of the second and last thirds is a nutrient foramen (h).

The distal end of the bone presents a smooth saddle-shaped articular surface, which, in well-marked bones, is divided, when seen from the ventral surface, by a slight nearly median ridge into two unequal portions, lateral and medial (i and j). The lateral half is rounded and is called the capitulum (i). It is broader ventrad than dorsad, and is not continued onto the dorsal surface of the bone. It is for articulation with the proximal end of the radius.

The medial half of the surface, the trochlea (j), is concave and passes directly into the capitular surface laterad, but is limited mediad by a sharp ridge. It is continued onto the dorsal surface of the bone, where it is limited also laterad by a ridge. It articulates with the semilunar notch of the ulna.

Proximad of the trochlea the dorsal surface presents a deep fossa, the olecranon fossa, which receives the olecranon of the ulna when the arm is straightened. On the ventral surface (Fig. 47) are two shallower fossæ separated by a longitudinal ridge. The one over the trochlea receives the coronoid process of the ulna when the arm is bent, and is called thence the coronoid fossa (l). The one over the capitulum, the radial[67] fossa (m), receives a triangular facet on the proximal end of the radius at the same time. Between the radial and coronoid fossæ on one side and the olecranon fossa on the other is only a thin plate of bone. On the medial surface of the distal end is a considerable roughened elevation, the medial epicondyle (n) (epitrochlea). It gives origin to flexor muscles and to the ulnar collateral ligaments of the elbow-joint. Opposite the medial epicondyle over the capitulum is the lateral epicondyle (o) for the origin of extensor muscles of the forearm and of the radial collateral ligaments of the elbow-joint. From the lateral epicondyle a ridge, the lateral supracondyloid ridge (p), continues proximad, curving onto the dorsal surface of the bone and ending about opposite the junction of the deltoid and pectoral ridges.

Proximad of the medial epicondyle the bone is pierced near its medial margin by an oblique oval foramen, the supracondyloid foramen (q).

Radius

(I, Figs. 49 and 50).—In the usual position the radius lies with its proximal end on the lateral side of the arm, articulating with the capitulum of the humerus. The proximal end is thus laterad of the proximal end of the ulna. Its distal end, however, lies on the medial side of the distal end of the ulna, so that the radius in the natural position crosses ventrad of the ulna.

The radius is a curved bone slightly flattened dorsoventrally, with enlarged ends. It may be described as consisting of a shaft and of a proximal and a distal end. Its proximal end presents on the ventral surface a tuberosity, the bicipital tuberosity (c), for the insertion of the tendon of the biceps muscle. Proximad of this the bone is contracted to form a neck (b) which is surmounted by a head (a). The head has on its proximal surface a depressed oval facet by which it articulates with the capitulum, and on its ulnar border a long narrow facet, the articular circumference (d), for articulation with the radial notch of the ulna; also a triangular facet (e), which fits into the radial fossa of the humerus.

The shaft is convex dorsad and concave ventrad. The distal end is somewhat pyramidal. From its medial or radial[68] side a wedge-shaped process, the styloid process (f), extends distad. The distal surface of the end together with the lateral surface of the styloid process form a concave articular cavity (g) which fits against the scapholunar bone.

Fig. 49.—Radius and Ulna, Dorsolateral View.

Fig. 50.—Radius and Ulna, Ventromedial View.

I, radius; II, ulna. a, head of radius; b, neck; c, bicipital tuberosity; d, articular circumference; e, facet for radial fossa of humerus; f, styloid process of radius; g, facet for articulation of scapholunar bone; h, semilunar (or great sigmoid) notch of ulna; i, coronoid process; j, olecranon; k, area for insertion of brachialis and clavobrachial muscles; l, rough area for attachment of interosseous membrane; m, styloid process of ulna.

The dorsal surface of the distal end is marked by longitudinal grooves for tendons, and its lateral or ulnar surface bears a concave facet for articulation with the ulna.

Ulna

(II, Figs. 49 and 50).—The ulna is a long slender[69] bone, flattened mediolaterad. It is enlarged at its proximal end and becomes gradually smaller toward the distal end.

The proximal end is marked ventrally by a deep excavation, the semilunar notch, or great sigmoid cavity (h). By the saddle-shaped articular surface of the semilunar notch it articulates with the trochlea. This articular surface is divided into two parts by a transverse non-articular area. The distal boundary of the semilunar notch is a blunt process, the coronoid process (i), which bears on its lateral surface a concave facet, the radial notch, for the head of the radius.

The portion of the bone proximad of the semilunar notch is called the olecranon (j). It fits into the olecranon fossa of the humerus when the arm is straightened, and is rough at its end for the insertion of tendons.

The body of the ulna becomes triangular distad. The distal end is slightly larger than the shaft just proximad of it, and bears on its radioventral side a hemispherical head for articulation with the radius. Distad of the head the bone continues as the flattened styloid process (m), which projects distad from its dorsolateral side and is smooth on the medial side of its apex, for articulation with the cuneiform bone of the wrist.

Carpus

(Fig. 51).—The carpus (wrist) consists of seven bones arranged in two rows, three in the proximal row and four in the distal row. Beginning on the medial side of the hand (thumb or radial side), the first bone in the proximal row is the scapholunar (a) (equal to the scaphoid or navicular and lunar of the human hand). It articulates with the radius. The next is the cuneiform (b), articulating with the styloid process of the ulna, and the next, which is attached to the cuneiform and projects freely ventrad, is the pisiform (c).

In the distal row the bone on the radial side is the trapezium (d); the next is the trapezoid (e), the next the os magnum (f), and the last the unciform (g). The distal row articulates with the metacarpals or bones of the palm of the hand (1-5).

In the kitten the scapholunar is represented by three bones, the scaphoid or navicular, on the radial side, the lunare, between the scaphoid and the cuneiform, and a centrale, which lies distad of the other two.

[70]

Scapholunar Bone. Os scapholunaris (Fig. 51, a).—The scapholunar is a quadrangular bone with the ventroradial angle produced into a blunt process. Its proximal surface is smooth and articulates with the distal end of the radius. The distal end is marked by oblique ridges and articulates with the unciform, os magnum, trapezoid, and trapezium. The ulnar surface articulates with the cuneiform, and the dorsal surface of the ventroradial process with the radial sesamoid.

Cuneiform Bone. (Os triquetrum BNA) (Fig. 51, b).—The cuneiform bone has the form of a flattened pyramid. Its base articulates with the unciform, its proximoulnar surface with the pisiform except at its dorsal margin, where it articulates with the styloid process of the ulna. On its proximoradial surface is a smooth facet for articulation with the scapholunar.

Pisiform Bone. Os pisiforme (Fig. 51, c).—The pisiform bone is about twice as long as broad, with enlarged ends. Its dorsal end articulates with the cuneiform, and on its proximal surface, separated from the above by a smooth ridge, is a smooth facet for articulation with the styloid process of the ulna.

Fig. 51.—Carpus, Metacarpus, and Phalanges, Dorsal Surface.

a, scapholunar bone; b, cuneiform; c, pisiform; d, trapezium; e, trapezoid; f, os magnum; g, unciform; h, radial sesamoid; i, proximal phalanges; j, second phalanges; k, distal phalanges; 1, 2, 3, 4, 5, metacarpals in order from the radial side.

Unciform Bone. (Os hamatum BNA) (Fig. 51, g).—The unciform is a wedge-shaped bone with the apex of the wedge directed proximad, and smooth for articulation with the scapholunar. By a part of its ulnar surface it articulates with the cuneiform, and by its radial surface with the os magnum. Its distal end articulates with the fourth and fifth metacarpals.

Os magnum. (Os capitatum BNA) (Fig. 51, f).—The os magnum may be described as an[71] oblong plate bearing on its proximal surface a semicircular ridge which crosses it diagonally. The proximal end of the bone articulates with the scapholunar. Its distal end articulates with the third metacarpal except near its ventroulnar angle, where it articulates with the fourth metacarpal. Its ulnar surface articulates with the unciform. Its radial border articulates with the trapezoid, the third metacarpal, and, by two facets, with the second metacarpal.

Trapezoid. (Os multangulum minus BNA.) (Fig. 51, e).—The trapezoid is somewhat wedge-shaped, with the apex of the wedge pointing ventrad. Its proximal side articulates with the scapholunar, its distal side with the second metacarpal, its ulnar side with the os magnum, and its radial side with the trapezium.

Trapezium. (Os multangulum majus BNA.) (Fig. 51, d).—The trapezium has the form of a triangular prism curved into a semicircle. The convex face looks proximad and articulates by its ventral half with the scapholunar. The ulnar surface articulates with the second metacarpal dorsally, and ventrally with the trapezoid. Its radial surface articulates with the first metacarpal.

Bones of the Hand or Manus

(Fig. 51, 1-5).—The Metacarpals. Metacarpus.—The metacarpals are the five bones of the palm of the hand; they are numbered from one to five, beginning with the thumb. They are cylindrical elongated bones with enlarged ends. The distal end is called the head, and the proximal end the base. Each head bears a hemispherical articular facet which is marked over its ventral half by a prominent smooth ridge. The surface dorsad of the ridge articulates with the proximal head of a phalanx. The ridge and the surface at its sides are for a pair of sesamoid bones.

The first metacarpal (1) is the shortest. Its head is oblique, and it articulates by the ulnar half of its proximal surface with the trapezium (d); by the radial half with the radial sesamoid (h).

The second metacarpal (2) is marked on the proximal part of its dorsal surface by an oblique groove passing from the radial side distad to the ulnar side. The base articulates with the trapezoid (e). The ulnar surface of the proximal end[72] articulates with the os magnum (f) and third metacarpal, while the radial surface articulates with the trapezium (d).

The third metacarpal (3) is the longest, and its base is rhomboidal with a projecting dorsoradial angle separated by a groove from the rhomboid surface. The proximal end articulates with the os magnum (f) and second metacarpal; the radial surface of the proximal end with the second, and the ulnar surface with the fourth, metacarpal.

The fourth metacarpal (4) has a base similar in form to that of the third, and when placed in position with the fifth the two form a hemispherical facet which articulates with the unciform (g) and os magnum (f). The fourth metacarpal articulates by its radial side with the third, and by its ulnar side with the fifth.

The fifth metacarpal (5) articulates by its proximal end with the unciform (g), and by the radial side of its proximal end with the fourth metacarpal.

Digits (Fig. 51).—The first digit of the hand is called the pollex (thumb), the second the index, the third the medius, the fourth the annularis, the fifth the minimus.

The first digit has two phalanges, each of the others three phalanges.

Phalanges (Fig. 51, i, j, k).—The phalanges of the proximal row (i) are elongated, flattened dorsoventrally and curved so as to be longitudinally convex dorsad. All have thickened ends. The proximal end is notched, and its proximal surface looks dorsad and is concave for the head of the metacarpal. The distal end is pulley-shaped, and the pulley surface extends farther on to the ventral than on to the dorsal surface, and serves for articulation with the middle phalanx.

The phalanges of the middle row (j) are like those of the proximal row, but shorter. The proximal surface is triangular and marked by a median facetted ridge. The whole surface is smooth and adapted to the distal end of the phalanx of the first row. The distal end is transversely elongated, so as to be cylindrical, and projects more toward the ulnar than toward the radial side. The distal phalanx (k) articulates with this cylinder so that when it is fully extended it lies on the ulnar side of the middle phalanx.

[73]

The distal phalanx (k) has the form of a quadrangular prism. It is excavated on its proximal surface for articulation with the middle phalanx. Its distal surface presents dorsad a deep excavation from the bottom of which arises a compressed plate of bone having the form of a bird’s beak. The depression receives the base of a claw, and the beak-like projection supports the claw.

Sesamoid Bones of the Hand. Ossa sesamoidea.—The hand contains, in addition to those already described, eleven small bones that are developed in tendons.

One of these, the radial sesamoid (Fig. 51, h), is closely applied to the radial end of the scapholunar bone. It is developed in the tendon of the extensor brevis pollicis muscle.

The other ten occur in pairs as small flattened curved bones on the ventral side of the joint between each metacarpal and the phalanx with which it articulates.

JOINTS AND LIGAMENTS OF THE THORACIC LIMBS.

The shoulder-joint is an arthrodial or ball-and-socket joint. The bones entering into its formation are the scapula and the humerus.

The capsular ligament or articular capsule is very ample and allows for extended movement of the humerus. It is attached to the edge of the glenoid fossa of the scapula and passing distad covers the head of the humerus and is inserted at the line of junction of the shaft and the epiphysis which forms the head of the bone. On the lateral side of the ventral surface of the humerus the attachment continues distad about two centimeters along the lateral edge of the bicipital groove. On the medial side the insertion passes over the proximal end of the lesser tuberosity. A strong transverse band passes from the greater tuberosity to the lesser tuberosity and bridges the bicipital groove, converting it into a canal. The lateral and medial parts of the capsule are strengthened by thicker bands of fibres, the more prominent medial one of which passes from the coracoid process of the scapula to the lesser tuberosity. To the capsule are closely united parts of the supraspinatus,[74] infraspinatus, coracobrachialis, and subscapularis muscles. A synovial membrane lines the capsule within and forms a sheath around the biceps tendon, so that the latter does not actually enter the synovial capsule.

The elbow-joint (Figs. 52 and 53) is a ginglymus or hinge-joint. The bones which enter into it are the humerus, radius, and ulna.

The capsule of the joint forms a sac, with the following attachments to the bones: (1) To the humerus it is attached at the proximal edge of the coronoid and radial fossæ; to the sides of the capitulum and trochlea distad of the two epicondyles, and to the distal edge of the olecranon fossa. (2) To the ulna it is attached at the edges of the radial and semilunar notches; (3) to the radius around the articular facet, two or three centimeters distad of the border. Many of the muscles of this region are closely attached to the capsule.

Fig. 52.—Ligaments of Elbow-joint, Medial Side.

Fig. 53.—Elbow-joint, Lateral View.

Fig. 52.—1, humerus; 2, ulna; 3, radius. a and b, the two medial collateral ligaments.

Fig. 53.—1, humerus; 2, radius; 3, ulna. a, dorsal collateral ligament; b, ventral collateral ligament; c, annular ligament.

Closely connected with the capsule of the joint are the collateral ligaments. The two medial collateral ligaments (Fig. 52) arise from the medial epicondyle. One (b) passes distad and laterad to the interval between the radius and ulna; here it divides, one branch going to the head of the radius, while the other is attached to the lateral surface of the ulna at the edge of the semilunar notch. The second medial ligament (a) lies dorsad of the first; it passes to the medial surface of the ulna, at the distal edge of the semilunar notch.

[75]

The two collateral ligaments on the lateral side (Fig. 53) arise from the lateral epicondyle. The ventral one (b) passes almost directly distad and is inserted into the lateral surface of the proximal end of the radius about one centimeter from the articular surface; its inner surface is partly united to the annular ligament (c) of the radius. The dorsal one (a) is attached to the lateral border of the semilunar notch of the ulna.

Articulations of Radius and Ulna.—The proximal radio-ulnar articulation (Fig. 53) is by a pivot-joint or trochoid. The two bones are held in place by the annular ligament (Fig. 53, c). This is attached on the lateral side to the dorsal border of the radial notch of the ulna, passes around the head of the radius, receiving some ligamentous fibres which come from the lateral epicondyle, and is attached to the coronoid process of the ulna. The annular ligament is closely united with the capsule of the joint.

The radius and ulna are united for about their middle third by the thin interosseous membrane, which fills the interosseous space between their adjacent edges.

The Wrist.—At the wrist or carpus there are in reality three joints, the first between the radius and ulna proximad and the first row of carpals distad, the second between the two rows of carpal bones, the third between the distal row of carpals and the metacarpals. The first two are movable joints; the third is not. Each of these three joints has a capsule, and the bones entering into the joints are interconnected by numerous ligaments. These ligaments are named by combining the names of the two bones which they interconnect. Ligaments which interconnect bones of the same row in the carpus are sometimes distinguished as interosseous ligaments, as contrasted with intercarpal ligaments, which connect together bones of different rows. According to their position the ligaments may also be distinguished as dorsal, ventral, and lateral. Detailed descriptions and figures of all these ligaments are given by Strauss-Durckheim.

Metacarpals.—The joint between the carpals and metacarpals has been described. At the distal end of the metacarpals the articulations with the phalanges have each a[76] capsule. The joint is further strengthened by a double lateral ligament on each side. The two sesamoid bones at each joint are interconnected by a strong transverse ligament, and each is connected with the head of the metacarpal and the base of the first phalanx by a lateral ligament.

Phalanges.—Between the phalanges the joints possess capsules, and each has a radial and an ulnar lateral ligament.

VI. BONES OF THE PELVIC EXTREMITIES.

Innominate Bones. Ossa innominata. (Os coxæ BNA)

(Figs. 54 and 55).—The two innominate bones articulate with the sacrum and extend thence caudoventrad and finally turn mediad and unite in the middle line, forming the symphysis pubis. They thus form an arch, the pelvic arch, pelvic girdle or pelvis, which is closed dorsad by the sacrum.

In the middle of the lateral surface of each bone is a hemispherical depression, the acetabulum (Fig. 55, d), which receives the head of the femur.

Fig. 54.—Innominate Bone of Kitten, Ventrolateral View.

I, ilium; II, ischium; III, pubis; IV, acetabular bone. a, acetabulum; b, obturator foramen.

In the kitten each innominate bone is composed of three principal parts united by sutures (Fig. 54). From the sacrum to the acetabulum is a single bar, the ilium (I). Caudad of the acetabulum are two bars. The dorsal one of these is the ischium (II), and the ventral one is the pubis (III). The ischium enters into the formation of the acetabulum (a), but the pubis does not. The two bones, however, are in contact at the ventral edge of the acetabulum. From this point they diverge, but unite with one another again near the middle line and thus enclose an oval foramen, the obturator foramen (b). Wedged between the ilium, ischium, and pubis at their point of junction and helping to form the acetabulum is a small irregular bone, the acetabular bone (IV). In the adult cat these four parts are united into a single bone which is nevertheless usually described, ignoring the acetabular[77] piece, as made up of ilium (Fig. 55, I), ischium (II), and pubis (III).

Fig. 55.—Innominate Bone of Adult Cat, Ventrolateral View.

I, ilium; II, ischium; III, pubis. a, crest of the ilium; b, posterior inferior spine; c, great sciatic notch; d, acetabulum; d′, incisura acetabuli; e, spine of the ischium; f, lesser sciatic notch; g, tuberosity of the ischium; h, ramus of the ischium; i, ramus of the pubis; j, obturator foramen; k, pubic tubercle; l, iliopectineal line; l′, ilio-pectineal eminence; m, anterior superior process.

The ilium (I) is somewhat contracted at the middle and broader at its ends. One end enters into the acetabulum (Fig. 55, d) and forms about one-fifth the articular surface. This end is also the thickest part of the bone. The lateral surface of the ilium is concave for the attachment of muscles. The medial surface is smooth over its acetabular half and rough over its sacral half. The rough portion is marked at its junction with the smooth portion by the ear-shaped auricular impression by which the bone articulates directly with the sacrum. The caudal half of that part of the medial surface craniad of the auricular impression gives attachment to the ilio-sacral ligaments which bind the ilium to the sacrum. The dorsal border is straight at its cranial end and concave and rounded at its caudal end. Between the two portions and at the dorsal edge of the auricular surface is a protuberance corresponding to the posterior inferior spine (Fig. 55, b) of the human ilium. The concavity of the dorsal border (c) corresponds to the great sciatic notch of the human ilium. At its caudal end is the short spine of the ischium (e), which is not a part of the ilium. The ventral border of the ilium is broad caudad, becoming narrower craniad. The lateral margin of the ventral border is continued to a tuberosity at the edge of the acetabulum; its medial margin is called the[78] iliopectineal line (l) and extends on the pubis to the symphysis. An eminence, the iliopectineal eminence (l′), on the iliopectineal line, lies opposite to the acetabulum at about the junction of the ilium and pubis. The cranial end of the bone is thickened, forming the crest (a) of the ilium. At the junction of the crest with the ventral border is a projection, the anterior superior process (m) of human anatomy.

The pubis (III) (including the acetabular bone) enters into the formation of the acetabulum (d) constituting about one-sixth the circumference, but less than one-sixth its area. It may be described as a flat, curved bone, contracted at the middle and expanded at the ends. The dorsal end enters into the acetabulum; the ventral end unites with the opposite bone at the symphysis pubis and sends caudad a projection, the ramus (i) of the pubis, which unites with the ramus of the opposite bone to form about two-thirds of the entire symphysis. At the sides of the symphysis a slightly marked angle projects craniad from each of the pubic bones; these two together constitute the pubic tubercle (k), for the origin of the rectus abdominis muscle. The surfaces of the ramus are smooth. One of its borders is concave and enters into the formation of the obturator foramen (j). Another of its borders is the iliopectineal line (l). Its third border is rough for the symphysis.

The ischium (II) has the form of a triangular prism contracted at the middle. Its cranial end forms nearly two-thirds of the acetabulum. Its caudal end bears dorsad a rough thickening, the tuberosity of the ischium (g). From the caudal half of the ventral border of the bone a sickle-shaped process, the ramus (h) of the ischium, curves medioventrad and then craniad and joins the ramus of the pubis. Its medial border is rough and enters into the symphysis, forming the caudal one-third. The lateral angle of the bone is rounded. Its dorsal angle is marked near the cranial end by the spine (e) of the ischium. The concavity between this spine and the tuberosity corresponds to the lesser sciatic notch (f) of human anatomy.

The acetabulum (d) is cup-shaped. The ventral one-sixth of its border is deficient and a broad groove extends from the[79] deficiency to the bottom of the cup. The deficiency, incisura acetabuli, or acetabular notch (d′), is closed naturally by a ligament, and the groove gives origin to the ligament (ligamentum teres) which attaches the head of the femur.

Fig. 56.—Left Femur, Ventral Side.

a, head; b, neck; c, depression for round ligament; d, great trochanter; e, trochanteric fossa; f, intertrochanteric line; g, lesser trochanter; h, linea aspera; i, medial condyle; j, lateral condyle; k, intercondyloid fossa; l, lateral epicondyle.

Femur

(Fig. 56).—The femur is the proximal bone of the posterior extremity. It consists of enlarged proximal and distal ends connected by a nearly cylindrical shaft. The proximal end presents on its medial side a hemispherical head (a) which fits into the acetabulum. It is supported by a neck (b) which is contracted near the head and expanded dorsoventrally where it joins the remainder of the bone. The medial surface of the head presents near its ventral border a depression (c) for the insertion of the round ligament of the femur. Ventrad the articular surface of the head extends as an acute projection onto the shaft, so that the whole articular surface appears somewhat pear-shaped. On the lateral side of the proximal end opposite the head is a projecting mass, the great trochanter (d), forming the end of the shaft. On the medial side of the great trochanter at its junction with the neck is a deep fossa, the trochanteric fossa or digital fossa (e) for the insertion of muscles. From the ventral surface of the great trochanter a ridge, the intertrochanteric line (f), is continued distad, ending in a pyramidal projection, the lesser trochanter (g), which serves for the insertion of muscles. A second ridge is continued to the lesser trochanter from the neck. A slight but well-marked ridge, the spiral ridge or line, runs round two sides of the neck parallel to the second ridge.

[80]

The shaft is nearly straight and cylindrical. A rough line is continued along its ventral surface from the lesser trochanter, and a similar line along its lateral surface from the greater trochanter; these unite ventrad to form the linea aspera (h). On its ventral surface is a nutrient foramen, directed proximad.

The shaft gradually widens distad and ends in two condyles (i and j) which are continuous dorsad but separated ventrad by a deep notch, the intercondyloid fossa (k). The distal surface of the shaft and condyles is articular. This articular surface is larger on the lateral condyle (j). The part of the articular surface on the end of the shaft (patellar surface) is for the patella; that part of it on the condyles and separated by the notch is for the tibia.

On the lateral surface of the lateral condyle is a slight prominence, the lateral epicondyle (l), and on the medial surface of the medial condyle is another prominence, the medial epicondyle; both are for the attachment of ligaments.

Patella

(Fig. 1, r).—The patella is a small flat bone with a pear-shaped outline, having its apex distad. It lies against the articular surface at the lower end of the shaft of the femur. It thus covers the knee-joint. The inner surface is smooth and convex from side to side, but concave in a proximodistal line. It fits against the lower end of the femur. Its outer surface is rough and concave. It is a sesamoid bone inserted in the tendon of the quadriceps femoris muscle.

Three other sesamoid bones are found in the region of the knee (see Fig. 61, p. 89). Two are in the tendons of the gastrocnemius muscle, proximad of the two condyles of the femur. The third is in the tendon of the popliteus muscle, just laterad of the lateral condyle of the femur.

Fig. 57.—Tibia and Fibula of Left Leg, Dorsal View.

I, tibia; II, fibula. a, medial condyle of the tibia; b, lateral condyle; c, spine of the tibia; d, crest of the tibia; e, tubercle for attachment of the patellar ligament; f, medial malleolus; g, projection of dorsal surface of the tibia; h, head of fibula; i, lateral malleolus.

Tibia

(I, Fig. 57).—The tibia is the longer of the two bones of the leg between the knee and the ankle, and is the longest bone of the body. It has a triangular shaft and enlarged proximal and distal ends.

The proximal end is curved ventrad and projects into two prominences, the tuberosities, on either side. Each tuberosity bears on its proximal end an articular facet for the condyles of the femur; these are known respectively as the lateral and[81] medial condyles (a and b) of the tibia. The condyles are oval, convex dorsoventrad, and concave from side to side. The two condyles are separated at the middle of their contiguous margins by a bicuspid projection, the spine of the tibia (c). They are continuous dorsad, but separated ventrad by a deep notch between the tuberosities, the popliteal notch. On the distal side of the lateral condyle is an elongated facet for the proximal end of the fibula.

The shaft is triangular, smallest at about its middle and enlarged at both ends. It presents dorsal, medial, and lateral borders, and medial, lateral, and ventral surfaces. The lateral surface is concave proximad. The medial surface is convex. The two are continuous at the distal end. At their proximal ends the border separating them is raised into a prominent ridge, the crest (d) of the tibia, the proximal end of which contributes to increase the proximal surface of the bone, and presents an oblong tubercle (e) for the insertion of the ligamentum patellæ (ligament of the quadriceps femoris muscle). The ventral surface is concave proximad where it abuts upon the tuberosities. Its proximal half is crossed by two rough parallel lines, the distal one of which crosses in a spiral course from the lateral to the medial border; near its beginning is a nutrient foramen.

The distal end extends farther distad on its medial side. The extension is the medial malleolus (f). The malleolus presents two grooves on its medial surface for the tendons of muscles. On the lateral side of the distal end is an oblique triangular facet for the distal end of the[82] fibula. The ventral surface of the distal end presents an oblique border which passes from the apex of the malleolus proximolaterad. The dorsal surface extends into a V-shaped projection (g) between the malleolus and the fibular facet. The distal end presents an oblique ridge running from the apex of the V-shaped extension of the dorsal surface to near the base of the medial malleolus. The ridge and the concavities on either side of it, the medial one of which is deeper, fit against the proximal trochlear surface of the astragalus.

Fibula

(II, Fig. 57).—The fibula lies at the lateral side of the tibia in the shank. It is a slender triangular bone with enlarged proximal and distal ends.

The proximal end or head (h) is flattened. It bears a facet on its proximomedial surface for articulation with the tibia, and is longitudinally grooved on the outer surface.

The shaft has a very sharp medial border. This border is turned toward the tibia and gives attachment to the interosseous membrane, which runs between the tibia and fibula.

The distal end is expanded to form the lateral malleolus (i). This bears a facet on the proximal portion of its medial surface near its dorsal margin, for the tibia, and distad of this is a second facet for the astragalus. The ventral and lateral surfaces are grooved for tendons.

Tarsus

(Fig. 58).—The tarsus consists of seven bones. The longest of these, lying on the lateral side of the foot and forming the support of the heel, is the calcaneus or os calcis (a). It articulates distad with a bone, the cuboid (c), which bears the fourth and fifth metatarsals. Lying between the calcaneus and the tibia is the astragalus or talus (b), the distal end of which articulates with the boat-shaped navicular or scaphoid (d). The scaphoid bears on its distal surface the three cuneiform bones, lateral (e), medial (g), and intermediate, bearing the rudiment of the first metatarsal and the second and third metatarsals.

Astragalus. (Talus BNA) (Fig. 58, b).—The astragalus may be divided into body, neck, and head. The body is marked on its proximal surface by a deep pulley-like groove for the articulation with the distal end of the tibia, and on its[83] lateral and medial surfaces by curved facets for articulation with the malleoli of the tibia and fibula. This entire surface for articulation with the bones of the leg is known as the trochlea. The lower surface is marked by two facets separated by a groove; these are for articulation with corresponding facets on the calcaneus. Distally the bone contracts to form the neck and enlarges at the end, forming the head, which is smooth on its distal surface for articulation with the navicular or scaphoid.

Calcaneus (Fig. 58, a, and Fig. 59).—The calcaneus (os calcis) is the largest bone of the foot and forms the heel. It is two or three times as long as broad and has six surfaces: dorsal, ventral, medial, lateral, proximal, and distal. The proximal one-half of the dorsal surface (Fig. 59) is smooth, while the distal half is broadened and bears two facets which are separated by a groove. These articulate with the corresponding facets on the astragalus. The medial[84] facet is borne on a projection of the bone, the sustentaculum tali (c). Distad of the facets the surface is rough. The ventral surface is smooth. The proximal end (b) is grooved for the tendon of Achilles. The lateral surface is smooth and marked by a grooved tubercle, the peroneal tubercle (d), near the distal end. The medial surface is marked by part of the articular facet for the astragalus, and also by the grooved sustentaculum tali. The distal end (a) articulates with the cuboid.

Fig. 58.—Tarsus, Metatarsus, and Phalanges of Left Foot, Ventral View.

a, calcaneus; b, astragalus; c, cuboid; d, scaphoid; e, lateral cuneiform; g, medial cuneiform; h, peroneal groove, for the tendon of the peroneus longus muscle; i, proximal phalanges; j, second row of phalanges; k, distal phalanges; l, sesamoid bones. 1, rudimentary first (medial) metatarsal; 2, 3, 4, 5, the other metatarsals.

Fig. 59.—Calcaneus of Right Foot, Dorsal View.

a, distal facet for cuboid; b, proximal end with groove for tendon of Achilles; c, sustentaculum tali; d, peroneal tubercle.

 

Cuboid. Os cuboideum (Fig. 58, c).—The cuboid has somewhat the form of a cube and articulates by its proximal end with the calcaneus (a), and by its distal end with the fourth and fifth metatarsals. Its medial surface articulates with the scaphoid (d) and lateral cuneiform (e). The ventral surface is marked near its distal end by an oblique ridge, distad of which is a deep groove, the peroneal groove (h), for the tendon of the peroneus longus muscle.

Scaphoid. (Os naviculare pedis BNA) (Fig. 58, d).—The scaphoid is a boat-shaped bone. Its proximal surface is marked by a concave facet for the head of the astragalus (b), and its distal surface has three facets for the lateral (e), intermediate and medial (g) cuneiform bones. At the junction of the ventral with the medial surface is a prominent tubercle. The lateral surface bears two linear facets for articulation with the calcaneus (a) and cuboid (c).

Lateral Cuneiform. Ectocuneiform. Os cuneiforme tertium BNA (Fig. 58, e).—The lateral cuneiform is a wedge-shaped bone with a hooked process extending from the ventral sharp angle of the bone. It articulates by its proximal end with the scaphoid (d), and by its distal end with the third metatarsal. The medial surface bears near its distal end two facets for the second metatarsal, and on its proximal end a facet for the intermediate cuneiform. The caudal surface has a facet on its proximal end for the cuboid (c).

Intermediate Cuneiform. Mesocuneiform. Os cuneiforme secundum BNA.—The intermediate cuneiform is small and wedge-shaped, with the base of the wedge dorsad. It lies between the lateral cuneiform and the medial cuneiform, articulates by its proximal end with the middle facet of the scaphoid,[85] and bears on its distal end the second metatarsal. It is not visible in ventral view.

Medial Cuneiform. Entocuneiform. Os cuneiforme primum BNA (Fig. 58, g).—The medial cuneiform lies on the medial side of the foot. It is a flat triangular bone about twice as long as broad, and broader at its proximal end than at the distal end. It bears on its distal end the rudimentary first metatarsal. The proximal end is oblique and bears a concave facet for the lateral distal facet of the scaphoid (d). The lateral surface has a concave facet at its proximal end for the intermediate cuneiform, while the distal portion is applied against the medial surface of the second metatarsal.

Bones of the Foot or Pes

(Fig. 58).—Metatarsals. Metatarsus (Fig. 58, 1-5).—The metatarsals are five in number. They bear a close resemblance to the metacarpals, but they may be distinguished by their bases.

The first metacarpal (1) is rudimentary and conical. Its base has a facet for the distal end of the medial cuneiform (g), while the outer surface fits into a depression on the inner surface of the base of the second metatarsal.

The second (2). The proximal surface of the base is triangular, corresponding to the distal end of the intermediate cuneiform. The medial surface is marked by two concavities, one along the proximal border for the distal end of the medial cuneiform, and one distad of this for the first metatarsal. The lateral surface bears on the proximal margin an oblique triangular facet dorsad and a similar facet ventrad, both for the lateral cuneiform (e). Distad of these facets is a rough ridge.

The third (3). The proximal end of its base is a triangular facet with the apex directed ventrad and the sides excavated. It is for the distal end of the lateral cuneiform (e). Its medial surface presents a depression which receives the ridge of the second metatarsal. On the lateral surface a short distance distad of the proximal border is a triangular concave facet, and near the proximal border ventrad a second concave facet. Both are for the fourth metatarsal.

The fourth (4). The proximal end is convex, notched medially and facetted for the cuboid (c). Its medial surface[86] bears dorsad, a short distance from its proximal end, a smooth tubercle, and ventrad a small convex facet. Both articulate with facets on the lateral surface of the third metatarsal. The lateral surface has a sinuous facet along its dorsal border, and ventrad of this a depression. There is a second facet along the ventral border. Both facets are for the fifth metatarsal, and the depression is for ligaments.

The fifth (5) has its base flattened and expanded so as to be wedge-shaped, with the apex of the wedge directed proximad. Its dorsal end extends into a tubercle. It thus presents only lateral and medial surfaces. The medial surface shows two tubercles, one distad of the other. The distal tubercle and the distal half of the proximal tubercle are facetted and fit into the sinuous facet on the fourth metatarsal. A narrow facet on the ventral border of the surface articulates with the facet on the ventral border of the lateral surface of the fourth metatarsal. The proximal half of the distal tubercle is facetted for the cuboid (c). The lateral surface is smooth, non-articular, and obliquely grooved.

Phalanges (Fig. 58, i, j, k).—There are three phalanges in each of the four digits, and these are almost identical with those described for the manus.

Sesamoid Bones. Ossa sesamoidea (Fig. 58, l).—The sesamoid bones are found at the joints between the metatarsals and phalanges, and are in all respects like those of the manus.

JOINTS AND LIGAMENTS OF THE PELVIC LIMBS.

Ligaments of the Pelvis.—The ilium and sacrum are articulated at the auricular facet of the ilium and the corresponding rough surface of the sacrum. The joint is an amphiarthrosis, permitting very little movement. A capsular ligament surrounds the articular surface, being attached to the bones about its circumference; it is short and strong. Craniad of the capsule is a thick very short ligament, composed of very strong transverse fibres passing from the rough surface of the sacrum to the corresponding rough surface of the ilium. This forms the lateral iliosacral ligament, which is united at its caudal border to the capsule.

A strong, wide ligamentous band passes from the dorsal[87] border of the ilium to the sides of the sacrum. This is indistinctly subdivided into several bands, which together represent the long and short posterior iliosacral ligaments of man.

Symphysis pelvis.—The medial borders of the pubis and ilium meet in the middle line ventrad of the pelvis and are here united by cartilage. The joint is strengthened by numerous small bands which pass across the line of junction from one side to the other; these occur on both surfaces.

The Hip-joint.—The hip-joint is an enarthrosis, or ball-and-socket joint in which more than half the spherical head of the femur is received into the acetabulum. The depth of the acetabulum is increased by a rim of fibrocartilage about its margin, forming the labrum glenoidale. This passes across the acetabular notch, forming the transverse ligament of the acetabulum; beneath it blood-vessels and nerves pass into the acetabular cavity.

The capsule of the joint is large and loose. It is attached about the margin of the acetabulum, and passes over the head of the femur, to be attached to the bone several millimeters distad of the head. It thus encloses both the head and the neck of the femur.

The ligamentum teres, or round ligament, is a very strong, short ligament which passes from the depression in the head of the femur to the bottom of the acetabulum.

The Knee-joint (Figs. 60 and 61).—The joint between the femur and the tibia is very complex. The surfaces of the condyles of the femur do not correspond to those of the condyles of the tibia. Between the ends of the two bones are placed two disks of cartilage, the menisci, or semilunar cartilages (Fig. 60, c and d; Fig. 61, a and b), of such a form that the congruity of articular surfaces is restored. Each meniscus has a proximal surface corresponding to the form of one of the condyles of the femur, and a distal surface corresponding to a condyle of the tibia. The menisci are held in position by ligaments. The knee-joint permits not only backward and forward movement, but also a small amount of rotary motion.

The joint has two capsules, one on the dorsal (convex) side, the other on the ventral side. The two communicate[88] only by a small passageway lying within the joint between the ends of the bones. The dorsal one is attached to the femur several millimeters proximad of the patellar surface and some distance on each side of the latter. The patella is imbedded in its outer wall, and it is attached to the tibia on the edges of the articular surface of the latter, from the crest to the tuberosities. The capsule is also attached laterally and medially to the sides of the menisci, and is closely united to the patellar ligaments. Its cavity contains a mass of yellow fat.

The ventral capsule is attached to the borders of the articular surfaces of the femur and tibia on their ventral sides, to the menisci, and to the epicondyles of the femur and the tuberosities of the tibia. Its walls are stronger and its cavity smaller than those of the dorsal capsule.

The ligaments of the knee-joint (Figs. 60 and 61), aside from the capsules, may be classified into: (1) those which are connected with the patella; (2) collateral ligaments (Fig. 60, i and j), which pass from the epicondyles directly distad along the sides of the joint to the tibia or fibula; (3) crucial ligaments (Fig. 60, g and h; Fig. 61, c and d), which cross within the joint from one side of the femur to the opposite side of the tibia; (4) ligaments which hold the menisci in place (Fig. 60, e and f; Fig. 61, e and f).

(1) Ligaments of the Patella.—The patella is imbedded in the dorsal wall of the dorsal capsule of the joint. From its distal end a strong tendon or ligament passes distad to the crest of the tibia. This is known as the ligamentum patellæ: it may be considered a part of the tendon of M. quadriceps femoris. On the lateral side the capsule of the joint is strengthened by the transverse fibres of the tendon of M. plantaris, which aid in holding the patella in place.

(2) Collateral Ligaments.—Of these there are two. The ligamentum collaterale fibulare (Fig. 60, j) is attached to the lateral epicondyle of the femur and passes distad across the tendon of the plantaris muscle to the head of the fibula. Dorsad of the fibular ligament and parallel with it passes the tendon of origin of the extensor longus digitorum. The ligamentum collaterale tibiale (Fig. 60, i; Fig. 61, g) begins on[89] the medial epicondyle of the femur and passes distad to the lateral tuberosity of the tibia; part of it passes one to one and a half centimeters distad of the tuberosity to be attached to a rough ridge on the side of the tibia.

Fig. 60.—Knee-joint, from the Dorsal or Patellar Side.

Fig. 61.—Knee-joint, from the Ventral or Flexor Side.

Fig. 60.—The patella has been removed. 1, femur; 2, tibia; 3, fibula. a, patellar surface of femur; b, tubercle for attachment of ligamentum patellæ; c, medial meniscus; d, lateral meniscus; e, f, ligaments of the menisci; g, ligamentum cruciatum anterius; h, ligamentum cruciatum posterius; i, ligamentum collaterale tibiale; j, ligamentum collaterale fibulare.

Fig. 61.—The capsule of the joint has been opened. 1, femur; 2, tibia (fibula not shown); 3, sesamoid bone in lateral head of M. gastrocnemius; 4, sesamoid in medial head of M. gastrocnemius; 5, tendon of M. popliteus, with sesamoid bone; 6, 7, lateral and medial condyles of the femur, with the intercondyloid notch between them; 8, 9, lateral and medial condyles of the tibia, with the popliteal notch between them; a, b, medial and lateral menisci; c, ligamentum cruciatum anterius; d, ligamentum cruciatum posterius; e, f, ligaments of the lateral meniscus; g, ligamentum collaterale tibiale.

(3) Crucial Ligaments.—There are two of these also. The ligamentum cruciatum anterius, or anterior crucial ligament (Fig. 60, g; Fig. 61, c), is a thick, strong ligament which begins on the dorsal part of the proximal end of the tibia nearer the medial side (Fig. 60, g), and passes ventrad and proximad, between the ends of the two bones forming the joint, into the intercondyloid fossa of the femur, and becomes attached to the medial surface of the lateral condyle of the femur (Fig. 61, c). It is composed of two partially separated bands, forming a slight angle with one another. It is crossed near its dorsal and ventral ends by two of the ligaments of the menisci. The ligamentum cruciatum posterius, or posterior crucial ligament[90] (Fig. 60, h; Fig. 61, d), begins on the tibia at the edge of the popliteal notch (Fig. 61, d), nearer the medial side, and passes dorsad and proximad to be attached to the ventral edge of the patellar surface of the femur, in the intercondyloid fossa (Fig. 60, h).

(4) Ligaments of the Menisci.—There are five of these, connecting the menisci with the femur or tibia. One (Fig. 60, e) passes from the dorsal edge of the medial meniscus (c) transversely across the anterior crucial ligament (g) to the proximal end of the tibia nearer the lateral side. A second (Fig. 60, f) passes from the dorsal margin of the lateral meniscus (d) transversely beneath the anterior crucial ligament (g) to the proximal end of the tibia nearer the ventral side and medial border. A third (Fig. 61, f) passes from the ventral margin of the lateral meniscus obliquely across the anterior crucial ligament (c) to the lateral side of the medial condyle of the femur. The fourth (Fig. 61, e) is small, passing from the medial angle of the ventral border of the lateral meniscus distad to the popliteal notch. The fifth passes from the ventral border of the medial meniscus laterad beneath the posterior crucial ligament to the proximal end of the tibia, nearer the ventral and lateral sides.

Articulations between the Tibia and Fibula.—At the proximal end the fibula is as it were suspended from the distal side of the overhanging lateral tuberosity of the tibia by strong ligamentous tissue. The capsule of the joint is formed by an extension of the capsule of the knee-joint which passes between the tibia and fibula on the ventral side. Forming the dorsolateral wall of this extension is a strong, thick ligament which passes directly from the head of the fibula to the lateral surface of the lateral tuberosity of the tibia. A second more delicate ligament passes from the head of the fibula dorsoproximad to the tubercle laterad of the crest of the tibia, bridging a groove through which passes the tendon of M. extensor longus digitorum.

The tibia and fibula are connected throughout their length by an interosseus membrane. This is broad and very thin in its proximal part, narrower and thicker distad.

[91]

Distad the two malleoli forming the ends of the fibula and tibia are closely and immovably united. The capsule of the joint is here an extension proximad of the capsule of the articulation with the astragalus. On the dorsal side a short broad band of strong fibres passes from the surface of the tibia obliquely laterodistad to the border of the fibula; this forms the anterior ligament of the lateral malleolus. On the ventral side a very much weaker set of fibres forms the ventral wall of the articular capsule; it is called the posterior ligament of the lateral malleolus. Tibia and fibula are also connected on the dorsal side by the ligamentum transversum cruris, or transverse ligament of the lower leg (Fig. 91, 5), which spans the tendons of Mm. extensor longus digitorum and tibialis anterior. From the middle of the distal margin of this a slender supporting ligament passes distad and is inserted on the dorsal surface of the tarsus. The grooves in the two malleoli for the passage of the tendons are spanned by ligamentous fibres (retinacula) for holding the tendons in place.

Articulation between the Leg and the Foot.—At the distal end there is formed between the two malleoli a deep irregular fossa, into which is received the trochlea of the astragalus. The joint is covered by a large articular capsule, which passes also, as above noted, between the tibia and fibula. In addition to the capsule the following ligaments may be distinguished: (a) On the lateral side, (1) a short ligament from the fibula to the astragalus, directed toward the proximal end of the foot; (2) a ligament from the fibula to the calcaneus, attached to the latter proximad of the peroneal tubercle; (3) a stronger ligament from the fibula to the calcaneus, lying beneath the last-mentioned and directed toward the proximal end of the bone. (b) On the medial side may be distinguished (1) a strong ligament from the tibia (medial malleolus) to the sustentaculum tali, and passing thence onto the scaphoid; (2) a short ligament from the medial malleolus to the astragalus.

The Tarsus.—The articulations between the separate bones of the tarsus and between the tarsus and metatarsus have a considerable number of variously communicating articular capsules. The separate bones are connected by many ligaments.[92] The ligaments of the ventral surface (ligamenta plantaria) are especially well developed. Here may be noticed particularly a very large calcaneocuboid ligament, and a large calcaneocuneiform ligament which passes from the sustentaculum tali to the medial cuneiform. Those on the dorsal surface (ligamenta dorsalia) are less strong and numerous. Many ligaments, longitudinal, transverse, and oblique, pass also between the separate bones (ligamenta interossea). Ligamenta lateralia, on the lateral and medial borders of the foot, are also distinguishable. Strauss-Durckheim enumerates ninety-four ligaments of the tarsus; an account of each of these does not form part of the plan of the present work.

The ligaments of the metatarsus and phalanges are of the same general character as in the forelimb. Of these Strauss-Durckheim enumerates thirty-six; they will not be described here.

[93]

THE MUSCLES.

I. THE MUSCLES OF THE SKIN. (Fig. 62.)

M. cutaneus maximus (Fig. 62, b).—This is a very large, thin muscle which covers almost the whole side of the body. It arises from the outer surface of the latissimus dorsi (Fig. 68, m) near its ventral end and from the bicipital arch (Fig. 65, t′) in the axilla; from the linea alba for a considerable distance (two or three inches) caudad of the base of the xiphoid process, and from the thorax over a line joining the axilla and the base of the xiphoid. Sometimes a few fibres take origin from the fascia which covers the pectoantibrachialis on the ventral side of the arm.

From their origin the fibres diverge. The cranial ones curve about the base of the forelimb and are inserted into the skin at or near the middle line caudad of the first thoracic vertebra. The most cranial fibres of all are inserted about one to one and a half centimeters from the middorsal line; thence the line of insertion approaches the middorsal line to reach it at about the eighth or ninth thoracic vertebra. The middle fibres run parallel to the middorsal line in the lumbar and sacral regions as far as the root of the tail, a small bundle passing onto the dorsal side of the tail, another onto the ventral side. The fibres of the caudal portion pass onto the thigh, the ventral ones running in the fold of skin which stretches from thigh to abdomen, and are finally lost in the fascia along a line connecting the knee and the root of the tail. A strong fascia connects the adjacent borders of the cutaneus and platysma and lies over the scapular region.

[94]

Fig. 62.—Muscles of the Skin.

a, a′, a″, M. platysma; b, M. cutaneus maximus; c, M. orbicularis oris. Between the platysma and the cutaneus maximus are seen the following deeper muscles: d, M. acromio-trapezius; e, M. levator scapulæ ventralis; f, M. spino-deltoideus; g, M. acromio-deltoideus; h, M. clavobrachialis; i, caput laterale of M. triceps brachii; j, caput longum of M. triceps brachii; k, M. xiphi-humeralis; l, M. pectoralis minor. 1, external jugular vein.]

[95]

This muscle lies immediately beneath the integument. It covers the first layer of body muscles.

Action.—Moves the skin.

M. platysma (Figs. 62 and 64, a, a′, a″).—This muscle forms a thin layer of fibres covering the sides of the neck and face, in close relation with the integument. Several more or less distinct portions may be distinguished.

(1) Most of the fibres (a) arise from the middorsal line, from the occiput to the first thoracic vertebra, in a narrow fascia common to the muscles of the two sides. The most cranial fibres of this region arise as a small bundle from the external occipital crest, beneath the levator auris longus (Fig. 63, g, g′).

From this origin in the middle line the fibres pass craniolaterad. The most cranial fibres curve about the ventral side of the ear and pass toward the caudal angle of the eye, where they unite with fibres of the zygomaticus (Fig. 64, d) or corrugator supercilii lateralis (Fig. 64, k), or pass to the lower eyelid. Caudad of these the fibres cover the side of the face and become lost among the facial muscles, some passing to the lower eyelid, some to the fibrous pad which supports the vibrissæ, some to the angle of the mouth, some to the lower lip. The most ventral fibres meet the fibres of the opposite muscle just ventrad of the symphysis of the mandible.

The ventral free border of the platysma is separated on the ventral side of the neck from the border of the opposite muscle by a wedge-shaped area having its point at the symphysis menti.

The fibres of this portion of the muscle are interrupted by an attachment to the skin, along a line passing from the base of the ear to about the middle of the coracoid border of the scapula, The dorsal (a) and ventral (a′) portions of the muscle, separated by this line of attachment, are sometimes described as separate muscles (the supercervicocutaneus and cervicofacial, respectively, of Strauss-Durckheim).

(2) A band of fibres one or two centimeters across (a″) arises in the fascia of the side of the neck just craniad of the middle of the coracoid border of the scapula, and passes caudoventrad[96] toward the manubrium, its fibres crossing the fibres of the first part of the platysma at right angles. These fibres become lost in the fascia ventrad of the manubrium, or pass across the middle line to intermingle with the corresponding fibres of the opposite side. This portion of the platysma is sometimes absent.

The platysma is everywhere subcutaneous, except at its dorsocranial angle, where a small bundle of fibres is covered by the levator longus auris. It covers the deeper muscles of the neck and head. Closely attached to its inner surface are the submentalis and depressor conchæ, whose fibres bridge over the ventral interval between the borders of the platysmas of opposite sides.

Action.—Moves the skin of the face and neck.

II. THE MUSCLES OF THE HEAD.

A. Superficial Muscles.

—The most superficial layer of muscles on the face and head is formed by differentiation of the fibres of the platysma. The muscles thus formed are not clearly distinct from each other; in this region sets of fibres differing in direction and in origin or insertion receive separate names even though the different sets of fibres are closely interwoven.

In the quadrangle on the dorsal surface of the head enclosed between the two eyes and the two ears, a thin superficial sheet of fibres is found, in which a number of different sets may be distinguished (Fig. 63). These have received the following names.

M. intermedius scutulorum (Fig. 63, a).—This consists of a broad thin sheet of transverse fibres between the two external ears. The fibres are attached at either end to the scutiform cartilage (1) of the two ears, and pass without interruption across the middle line. At its cranial edge this muscle is continuous with the corrugator supercilii medialis (b); at its lateral edge with the frontoscutularis; at its caudal edge with the levator auris longus (g).

Relations.—Outer surface with the integument. Inner surface[97] with the galea aponeurotica (to which the muscle is closely united), the epicranius muscle (h) and the temporal muscle (n).

Action.—Draws the two ears dorsad, toward the middle line.

Fig. 63.—Muscles on the Dorsal Surface of the Head.

On the right side are shown the superficial muscles; on the left side the superficial muscles have mostly been removed, exposing the deeper muscles and the bone. a, M. intermedius scutulorum; b, M. corrugator supercilii medialis; c, M. orbicularis oculi; d, M. corrugator supercilii lateralis; e, cranial end fibres of M. platysma; f, M. adductor auris superior; g, g′, M. levator auris longus (g, cranial portion; g′, caudal portion); h, h′, M. epicranius (h, M. occipitalis; h′ M. frontalis); i, M. transversus auriculæ; k, M. auricularis superior (cut on the left side); l, M. abductor auris brevis; m, M. abductor auris longus; n, M. temporalis; o, cut origin of M. frontoscutularis; p, p′, M. levator labii superioris alæque nasi (p′, the origin from the maxillary bone); q, angular head of M. quadratus labii superioris. 1, scutiform cartilage; 2, external ear; 3, bones of the skull.

M. corrugator supercilii medialis (b).—This consists of a thin sheet of scattered fibres lying craniad of the last and intermingling with it. The fibres take origin near the middle line, pass laterad, then curve craniad, converging, to be[98] inserted into the whole extent of the upper eyelid, especially near the caudal angle. Here the fibres unite with those of the orbicularis oculi (c).

This muscle is continuous caudad with the intermedius scutulorum (a), craniad with the orbicularis oculi (c); laterad with the corrugator supercilii lateralis (d). Toward the medial side the fibres lose themselves in a tendinous sheet that joins the galea aponeurotica.

Relations.—Outer surface with the integument. Inner surface with the frontoscutularis and the skull.

Action.—Raises the upper eyelid.

M. orbicularis oculi (Fig. 63, c; Fig. 64, s).—This consists of two thin bands of muscle-fibres which lie one in either eyelid parallel to its border and unite at the angle of the eye.

Origin by short tendon-fibres from a tubercle on the surface of the frontal process of the maxillary bone just dorsad of the orbital end of the lachrymal canal and between the two parts of the quadratus labii superioris (Fig. 63, p and q).

The muscle splits into two parts which pass into the two eyelids. At the outer angle of the eye the two bands unite by the intervention of tendon-fibres between the muscle-fibre bundles of the two.

Relations.—Outer surface with the integument. Inner surface with the inner membrane of the eyelid.

Action.—Closes the eye.

M. corrugator supercilii lateralis (Fig. 63, d; Fig. 64, k).—This consists of a number of scattered fibres which arise from among the fibres of the frontoscutularis, and from the tendon lying just craniad of the external opening of the ear, to which are united also parts of the zygomaticus (Fig. 64, d) and submentalis (Fig. 64, c). The fibres pass craniad, converging so as to form a narrow band which is inserted at the caudolateral angle of the eye, where it unites with the orbicularis oculi (Fig. 64, s). This muscle is continuous on the medial side with the corrugator supercilii medialis (Fig. 64, j) and the frontoauricularis, on the lateral side with the platysma.

Relations.—Outer surface with the integument. Inner surface with the frontal bone and the frontoscutularis.

[99]

Action.—Pulls the angle of the eye caudad; at the same time pulls the external ear craniad.

M. frontoauricularis.—A few of the fibres which are attached along the upper eyelid sometimes pass dorsocaudad, mingled with fibres of the corrugatores supercilii medialis and lateralis and the frontoscutularis, to the craniomedial angle of the auricular cartilage, where they unite with the fibres of the adductor auris superior. These fibres are sometimes distinguished as the frontoauricularis muscle.

M. levator auris longus (Fig. 63, g and g′). (Part of the auricularis posterior of man.)—This lies on the caudal half of the dorsal surface of the head, forming a laterocaudal continuation of the intermedius scutulorum (a).

Origin from the middle line of the neck dorsad of the atlas, and from the sagittal crest for about one centimeter craniad of the external occipital tubercle. The fibres form a broad thin sheet which passes craniolaterad as far as the caudal end of the scutiform cartilage (1). Here the muscle divides; the major portion is attached to the scutiform cartilage (1), its fibres intermingling with those of the intermedius scutulorum (a). The caudal portion of the muscle (g′) passes onto the surface of the auricle, extending one or two centimeters distad; here it is inserted on an oblique line which lies directly craniad of the insertion of the transversus auriculæ (i).

The caudal portion of this muscle (g′) having origin above the atlas and insertion on the auricle, is sometimes considered a separate muscle, the supercervicoauricular or cervicoauricular. The cranial portion (g) with origin on the sagittal crest and insertion on the scutiform cartilage might be distinguished as the occipitoscutularis.

The levator auris longus is continuous at its cranial end medially with M. epicranius (h); laterally with M. intermedius scutulorum (a).

Relations.—Outer surface with the integument. Inner surface with the temporal muscle (n), the auricularis superior (k), the abductor longus auris (m), a narrow strand of the platysma, and the clavotrapezius.

Action.—Pulls the external ear dorsocaudad.

[100]

The four following muscles lie partly or entirely beneath those already described; they are doubtless formed as differentiations of the inner layers of the platysma.

M. auricularis superior (or attollens auris) (Fig. 63, k).—This muscle forms a band about one centimeter broad lying just beneath the levator longus (g).

Origin on the sagittal crest for about one centimeter craniad of the interparietal bone. The muscle passes laterad onto the surface of the auricle and is inserted into the auricular cartilage a little caudad of the middle of its convex surface. At its lateral end the cranial margin is closely united to the under surface of the levator auris longus (g).

Relations.—Outer surface at the medial end with the epicranius (h), the abductor auris longus (m), and levator auris longus (g); at the lateral end with the skin. Inner surface with the temporal muscle (n) and the auricular cartilage.

Action.—Pulls the external ear dorsad.

M. abductor auris longus (Fig. 63, m; Fig. 64, q). (Part of the auricularis posterior of man.)

Origin on the sagittal crest dorsad of the interparietal bone, caudad of that of the auricularis superior (k), which it partly covers.

The muscle passes laterad as a flat band 8 to 10 millimeters wide over the caudal surface of the concha of the ear, and is inserted (Fig. 64, q) on the lateral surface of the eminentia conchæ, caudad of the antitragus.

Relations.—Outer surface with the levator auris longus (g) and the integument. Inner surface with the auricularis superior (k), the abductor auris brevis (l), and the concha.

Action.—Pulls the external ear caudad.

M. abductor auris brevis (Fig. 63, l).

Origin by a tendon from the lambdoidal crest for one or two centimeters laterad of the middle.

The muscle passes lateroventrad lying beneath the abductor longus (m) as a flat band 6 to 8 millimeters wide which is inserted into the medial surface of the most proximal portion of the concha, just distad of its junction with the cartilaginous auditory meatus.

[101]

Relations.—Outer surface with the abductor longus (m), a small strand of the platysma and the concha. Inner surface with the temporal muscle (n).

Action.—Pulls the concha caudad.

M. epicranius (or occipitofrontalis) (Fig. 63, h and h′).

Origin on the sagittal crest or suture, just craniad of the origin of the levator longus auris, to the inner surface of which this muscle is closely united. From the origin the fibres pass craniad forming a band (h) about 8 or 10 millimeters in width, the two muscles lying close to one another and partly united in the middle line. About two centimeters craniad of their origin the fibres of both muscles end in a tendinous sheet, the galea aponeurotica, which covers the surface of the skull in the region between the ears and eyes, and is formed by the inner surface of the intermedius scutulorum (a) and other muscles of this region. The galea aponeurotica passes craniad onto the surface of the nose, where it gives origin again to a thin sheet of muscle-fibres (h′) which are inserted into the integument near the cranial ends of the nasal bones.

M. epicranius is thus formed of two muscular portions (h and h′), connected by a long tendinous sheet. The caudal portion is frequently distinguished as the occipital muscle (M. occipitalis, h), the cranial portion, on the nose, as the frontal muscle (M. frontalis, h′).

Relations.—Outer surface of the occipitalis (h) with the intermedius scutulorum (a), the levator auris longus (g), and the integument; inner surface with the auricularis superior (h), the abductor auris longus, and the bone. Outer surface of the frontalis (h′) with the integument; inner surface with the bones of the skull.

Action.—Moves the integument of the dorsal surface of the head and of the nose.

Ventrad of the external ear the following three differentiated portions of the platysma may be distinguished.

M. zygomaticus (major) (Fig. 64, d).—A slender band connecting the angle of the mouth with the scutiform cartilage of the ear.

Fig. 64.—Superficial Muscles on the Lateral Surface of the Head and Neck.

a, a′, a″, M. platysma; b, M. depressor conchæ; c, M. submentalis; d, M. zygomaticus (b, c, and d lie beneath the platysma and are seen through it); e, M. zygomaticus minor; f′, M. caninus; f, g, parts of M. quadratus labii superioris (f, angular head; g, M. levator labii superioris alæque nasi); h, M. myrtiformis; i, M. orbicularis oris; j, M. corrugator supercilii medialis; k, M. corrugator supercilii lateralis; l, M. adductor auris superior; m, M. helicis; n, M. antitragicus; o, M. adductor auris inferior; p, M. conchæus externus; q, M. abductor auris longus; r, parts of M. auricularis externus; s, M. orbicularis oculi.

At the angle of the mouth the fibres arise from among those[102] of the orbicularis oris (i), from both the upper and the lower lip. They pass thence dorsocaudad across the zygomatic arch as a band 8 or 10 millimeters wide which is attached to a tendinous aponeurosis lying just craniad of the external opening of the ear. To this aponeurosis are attached also, wholly or partly, the corrugator supercilii lateralis (k) and the submentalis[103] (c). The aponeurosis is united with the caudoventral angle of the scutiform cartilage and with the ventral edge of the cranial margin of the auricular cartilage.

This muscle is united at its cranial end with the orbicularis oris (i); throughout its length more or less with the platysma (a, a′); at its caudodorsal end with the corrugator supercilii lateralis (k) on its medial side, and with the submentalis (c) on its lateral side.

Relations.—Outer surface with the integument and the platysma (a′). Inner surface with the masseter, temporal, frontoscutularis, and adductor auris inferior (o).

Action.—Draws the angle of the mouth dorsocaudad; the external ear ventrocraniad.

M. submentalis (Fig. 64, c).—A flat band, similar to the last, and closely connected for some distance to its caudal border.

It arises as scattered fibres near the ventral middle line at about the level of the larynx, the fibres of the opposite muscles crossing the middle line and intermingling. The fibres pass dorsad, converging so as to form a narrower band, which unites dorsad with the zygomaticus (d), to be inserted with it into the tendinous aponeurosis above described.

Relations.—Outer surface with the integument and platysma (a′). Inner surface with the muscles of the hyoid, the digastricus, the parotid gland, the masseter, the zygomatic arch and the temporal muscle.

Action.—Draws the external ear ventrad.

M. depressor conchæ (Fig. 64, b).—A thin band of fibres, caudad of the last and parallel with it. The muscle arises as scattered fibres on the ventral surface of the neck, one or two centimeters craniad of the manubrium, the fibres of the muscles of opposite sides crossing the middle line and interdigitating. They pass dorsad, gathering together to form a narrow band which is inserted into the summit of the antitragus.

Relations.—Outer surface with the skin at the ventral end, and with the platysma (a′) further dorsad. Inner surface with the deep muscles of the neck and with the parotid gland.

Action.—Draws the external ear ventrad.

[104]

Lying deeper than the muscles thus far described, but closely connected with a number of them, are the following:

M. frontoscutularis. Origin (Fig. 63, o) on the frontal bone, along the supraorbital margin, from the craniomedial angle of the eye caudad to the zygomatic process of the frontal.

The fibres form a large muscle which passes dorsocaudad to the scutiform cartilage and is mostly attached along its ventrolateral border. Some of the outer fibres, however, pass distad of the scutiform cartilage toward the cartilage of the ear, thus joining the adductor auris superior (f).

The outer surface of this muscle is closely connected with fibres of the intermedius scutulorum (a), the corrugatores supercilii medialis (b) and lateralis (d), and the frontoauricularis, when this exists. The outer surface is covered near the origin by the orbicularis oculi (c), farther dorsad by the muscles just mentioned and the integument. The inner surface is in relation with the frontal bone and the adductor auris inferior (Fig. 64, o).

Action.—Pulls the ear craniad.

M. adductor auris inferior. (Fig. 64, o.) (Part of the auricularis anterior of man.)—A small muscle lying beneath the frontoscutularis.

Origin on the ligament which connects the zygomatic process of the frontal with the frontal process of the malar. The fibres form a thin band about 7 millimeters wide which passes craniad, closely united to the inner surface of the frontoscutularis. At the caudal edge of the zygomaticus (d) the muscle is interrupted by a short tendinous interval; the fibres then continue to their insertion on the tip of the antitragus.

Relations.—Outer surface with the frontoscutularis, the zygomaticus (d), and the integument. Inner surface with the temporal muscle and M. antitragicus (n).

Action.—Pulls the ear craniodorsad.

The remainder of the muscles of the ear, which, with the exception of the tragicus lateralis, merely interconnect the cartilages of the external ear or parts of these cartilages, are[105] described in connection with the account of the Auditory Organ.

Owing to the presence of the fibrous pad to which the whiskers are attached, the facial muscles between the eye and the mouth differ in the cat in some respects from those of related animals. This fibrous pad interrupts the muscles, frequently breaking muscles which are elsewhere single into two.

M. zygomaticus minor, or malaris (Fig. 64, e).—This muscle is not always present; when it exists it forms a very thin flat band of fibres passing from the ventral side of the eye toward the angle of the mouth.

Origin among the fibres of the orbicularis oculi (s) in the lower eyelid. The fibres pass ventrad; the insertion varies. In some specimens the insertion is among the fibres of the orbicularis oris (i), at the angle of the mouth, beneath the zygomaticus (d). In other cases this muscle is said to be inserted into the pad on which the whiskers rest.

Relations.—Outer surface with the integument and near its insertion with the zygomaticus major (d). Inner surface with the malar bone and the masseter muscle.

Action.—Pulls the angle of the mouth (or the whiskers) dorsad.

M. orbicularis oris (Fig. 64, i).—This forms a very thin layer of subcutaneous concentric fibres which surround the mouth, that of the upper lip being thicker than that of the lower lip. The part of the muscle in the lower lip has its fibres intermingled with those of the platysma (a′). In the median line the fibres of the upper lip are interrupted by a raphe, and caudad of this are intermingled with those of the caninus.

M. quadratus labii superioris (Fig. 63, p and q; Fig. 64, f and g).—This is a complex of muscle-fibres attached chiefly to the fibrous pad on which the whiskers rest. In it two parts can be distinguished more or less completely.

(1) M. levator labii superioris alæque nasi (Fig. 63, p; Fig. 64, g) on the sides of the nose. This arises (a) as a continuation of the frontal portion of the epicranius, and (b) in a[106] small bundle (Fig. 63, p) from near the dorsal end of the frontal process of the maxillary bone, in common with the next. The two slips thus formed unite craniad and are inserted partly into the integument on the outer side of the wing of the nose, partly into the skin of the upper lip, lateroventrad of the nose, and partly into the pad on which the whiskers are located, in connection with the next.

(2) The angular head (caput angulare), or levator labii superioris proprius (Fig. 63, q; Fig. 64, f) is much larger than the last. It takes origin from a small tubercle at the cranial border of the orbit, close to the origin of the orbicularis oculi. The fibres diverge from their origin to their insertion among the whiskers and in the fibrous pad on which the whiskers rest.

Action.—Erects the whiskers and raises the upper lip.

M. caninus or levator anguli oris (Fig. 64, f′).

Origin in a depression on the lateral surface of the maxillary bone, just craniad of the last premolar tooth. The fibres pass craniad, diverging, toward the whiskers, and have their insertion into the pad of tissue on which the whiskers rest.

Action.—Retracts the whiskers and raises the upper lip.

M. buccinator.—A thin muscle against the mucous membrane of the upper lip, partly united with the orbicularis oris and lying beneath M. caninus.

Origin on the outer surface of the superior maxillary bone in the depression between the infraorbital foramen and the first premolar tooth. The fibres pass ventrad, diverging, and mingle with those of the deep part of the orbicularis oris.

Relations.—Outer surface with the caninus and with the orbicularis oris. Inner surface with the mucous membrane of the upper lip.

Action.—Raises the upper lip.

M. myrtiformis (probably corresponds to a part of the human M. nasalis) (Fig. 64, h).—A thin sheet of muscle-fibres lying craniad of the levator labii superioris alæque nasi and passing from the whiskers to the nose and upper lip. It is distinguishable from the alæque nasi by the direction of the fibres.

[107]

Origin from the pad upon which the whiskers rest. The fibres diverge and have their

Insertion upon the wing of the nose and into the skin of the upper lip near the median line.

Action.—Dilator of the nares and elevator of the upper lip.

M. “moustachier” (Str.-D.) (probably corresponds to one of the Mm. incisivi).—It lies on the upper lip near the median line.

Origin from the outer surface of the premaxilla near the suture along the ventral border of the narial opening. The fibres diverge and pass caudad into the upper lip within the superficial fibres of the orbicularis oris.

Insertion.—The skin of the upper lip just craniad of the whiskers. Some of the fibres pass into the pad on which the whiskers rest.

Action.—It carries the lip craniad.

M. quadratus labii inferioris.—A thin flat band extending almost the length of the lower lip.

Origin from the alveolar border of the mandible, between the molar tooth and the canine. The fibres pass dorsad into the lower lip, where they intermingle with those of the orbicularis oris. The muscle is perhaps not constant.

Action.—Depressor of the lower lip.

Sometimes a few fibres arising from the cranial portion of the alveolar border of the mandibula pass mediad, seeming to join corresponding fibres from the opposite side. Such fibres would constitute a M. transversus menti.

B. Deep Muscles of the Head.

(a) Muscles of Mastication.

M. digastricus (Fig. 65, b).—The digastric is a thick prismatic muscle lying mediad of the angle of the lower jaw, connecting it with the base of the skull.

Origin by fleshy fibres from the outer surface of the jugular process of the occipital bone, and by a thin tendon from the tip of the mastoid process and from the ridge between the mastoid and the jugular processes. The muscle passes craniad, becoming at the same time broader and more voluminous.

[108]

Insertion.—The ventral border of the mandible craniad of the caudal border of the molar tooth. The insertion area extends slightly onto both surfaces of the bone and is triangular, with its apex craniad.

Relations.—Outer (ventral) surface with the following, beginning at the caudal end: the rectus capitis lateralis, the cleidomastoid (h), the submaxillary gland (2), the stylohyoid (d), a large lymph-gland (3), and the integument. Inner (dorsal) surface with the jugulohyoid, the tympanic bulla, the masseter (a), the hyoglossus, and the mylohyoid (c).

Action.—Depressor of the lower jaw.

M. masseter (Figs. 65 and 66, a).—The masseter forms the projecting mass so prominent in the cat behind and below the eye. Its outer surface is covered by a strong aponeurosis. The muscle is divided into three layers which are distinct as to origin, insertion, and direction of fibres, but which are otherwise not distinct.

The superficial layer takes

Origin by the superficial aponeurosis from the ridge which runs lengthwise of the outer surface of the malar bone, and by fleshy fibres from the surface of the malar bone ventrad of this ridge. The fibres pass obliquely caudad to their insertion.

Insertion.—The caudal half of the lateral margin of the ventral border of the mandible, by a sheet of fascia which lies on the inner surface of the muscle. The superficial fibres curve ventrad of the mandible and are inserted into a tendinous intersection between this muscle and the internal pterygoid (Fig. 66, c).

The middle layer takes

Origin from the ventral border of the malar bone, from opposite the molar tooth to within three millimeters of the mandibular fossa. The fibres pass ventrad and have their

Insertion into the mandible at the ventral border of the external coronoid fossa by means of the aponeurosis which covers the inner surface of the muscle.

Fig. 65.—Muscles on the Ventral Surface of the Thorax, Neck and Head.

The left side shows the first layer of muscles beneath the skin and skin-muscles; the right side shows the second layer, except in the head region. a, M. masseter; b, M. digastricus; c, M. mylohyoideus; d, M. stylohyoideus; e, M. sternohyoideus; f, M. thyreohyoideus; g, M. sternomastoideus; g′, M. sternothyreoideus; h, M. cleidomastoideus; i, M. levator scapulæ ventralis; j, M. clavotrapezius; k, M. clavobrachialis; l, M. pectoralis major; m, M. pectoantibrachialis; n, part of M. scalenus; o, M. pectoralis minor; p, M. xiphihumeralis; q, M. latissimus dorsi; r, M. epitrochlearis; s, caput longum of M. triceps brachii; t, M. biceps brachii; t′, bicipital arch; u, M. acromiodeltoideus; v, M. brachialis. 1, parotid gland; 2, submaxillary gland; 3, lymphatic glands; 4, external ear; 5, external jugular vein; 6, manubrium; 7, first rib; 8, common carotid artery; 9, molar gland.

The deep layer takes

Origin by a strong tendon from the ventral border of the zygoma just craniad of the mandibular fossa. The fibres pass cranioventrad, diverge and have their

[109]
[110]

Insertion into the external coronoid fossa.

Relations.—Outer surface with the following, beginning at the caudal border: the external ear, the parotid gland (Fig. 65, 1), the platysma, the submentalis, the zygomaticus (major), the zygomaticus minor. Ventral border with a lymph gland caudad of the angle of the mouth (Fig. 65, 3), and the digastric muscle (b). Inner surface with the bones.

Action.—A very powerful elevator of the lower jaw.

M. temporalis (Fig. 63, n).—The temporal muscle is the great mass taking origin from the temporal fossa and having its insertion on both surfaces and both borders of the coronoid process of the mandible. It may be divided into two layers, deep and superficial.

The superficial layer. The temporal fascia stretches over the temporal fossa, being attached to its borders; that is, to the sagittal and lambdoidal crests, to the curved ridge which connects the sagittal crest with the zygomatic process of the frontal bone, to the caudal border of this process, to the caudal border of the malar bone, to the caudal border of the ligament connecting the malar bone and the zygomatic process of the frontal, and to the dorsal border of the zygomatic process of the temporal bone and its dorsal root. The craniolateral part of this fascia is much stronger than the remainder.

The muscle-fibres take origin from the strong craniolateral part of the inner surface of the temporal fascia, from the groove on the temporal bone dorsad of the dorsal root of its zygomatic process, from the whole inner surface of this process and of the zygomatic process of the malar bone, and sometimes also from that part of the frontal bone which lies within the temporal fossa.

Insertion.--The outer surface of the coronoid process of the mandible dorsad of the coronoid fossa, and both borders of the coronoid process. The cranial fibres may be inserted craniad of the coronoid process onto the aponeurosis covering the deep portion of the muscle. The caudal part of this portion is more or less distinct and is sometimes described as a separate head.

The deep portion.

Origin by fleshy fibres from the whole surface of the temporal[111] fossa, except in cases where the cranial part of the fossa is occupied by the superficial portion. The fibres converge toward the coronoid process, and the ventral third of the outer surface is covered by strong fascia.

Insertion.—The whole inner surface of the coronoid process of the mandible.

Relations.—Outer surface with the external ear, the ear-muscles (Fig. 63), the epicranius (Fig. 63, h), the corrugatores supercilii lateralis (d) and medialis (b). Inner surface with the skull, the pterygoideus externus (Fig. 66, b), and the structures within the orbit.

Action.—Elevator of the lower jaw.

M. pterygoideus externus (Fig. 66, b).—This lies on the medial side of the mandible, ventrad of the ventral portion of the temporal.

Origin.—The external pterygoid fossa (Fig. 40, h), which lies on the lateral surface of the perpendicular plate of the palatine bone, and on the surface of the pterygoid process of the sphenoid bone dorsad of the internal pterygoid fossa. The fossa extends from the foramen rotundum to the sphenopalatine foramen. The muscle passes with parallel fibres laterad, ventrad, and caudad and has its

Insertion by a strong flat tendon into the medial surface of the mandible near its ventral border between the opening of the mandibular canal and the base of the angular process.

Relations.—Ventral surface with the masseter (a), the digastric, and the soft palate with its muscles. Dorsal surface with the temporal, and near its origin with the muscles of the eye, and other structures in the orbit.

Action.—Elevator of the lower jaw.

M. pterygoideus internus (Fig. 66, c).—This lies caudad of the pterygoideus externus, nearly parallel to it, and is connected with it at its lateral end.

Origin from the whole surface of the internal pterygoid fossa (Fig. 40, s).

Insertion by fleshy fibres into the ventral surface of the external pterygoid and its tendon, into the medial surface of the angular process of the mandible and caudad of the angular[112] process for about one centimeter into a fascia common to it and the masseter muscle, and into the stylomandibular ligament.

Fig. 66.—Obliquely Ventral View of the Deep Muscles of the Lower Jaw and Palate.

The tongue and other structures forming the floor of the mouth have been removed, showing the roof of the mouth. On the right side the masseter and pterygoideus internus have been removed, a, a′, M. masseter; b, M. pterygoideus externus; c, M. pterygoideus internus; d, d′, M. tensor veli palatini; e, e′, M. levator veli palatini; f, M. constrictor pharyngis superior. 1, aponeurosis of the velum palatinum; 2, ridges formed by the palatines and pterygoid portions of the sphenoid; 3, hamular processes; 4, free caudal border of the velum palatinum; 5, bulla tympani.

Relations.—Ventral surface with the styloglossus and the soft palate. Dorsal surface with the bulla tympani (5) and base of the skull. Cranial surface with the pterygoideus externus (b), with which it is partly united.

Action.—Assists the pterygoideus externus.

(b) Muscles of the Hyoid Bone.

M. stylohyoideus (Fig. 65, d).—The stylohyoid is a ribbon-like muscle about three millimeters wide. It lies on[113] the superficial surface of the digastric, caudad and ventrad of the angle of the jaw.

Origin by fleshy fibres from the outer surface of the stylohyal bone near its middle. The muscle curves over the outer surface of the digastric (b), and ends about one centimeter from the middle line in a slender flat tendon.

Insertion into the middle of the ventral surface of the body of the hyoid bone. Some of the fibres of the mylohyoid (c) are inserted into its tendon.

Relations.—Outer surface with the submaxillary gland (2) and the submentalis muscle. Inner surface with the digastric (b), the hyoglossus (Fig. 67, h), and the geniohyoid (Fig. 67, g).

Action.—Raises the hyoid.

M. geniohyoideus (Fig. 67, g).—The geniohyoid is a long slender muscle which lies next to the median line between the symphysis menti and the hyoid bone, closely applied to its fellow of the opposite side and within (dorsad of) the mylohyoid (Fig. 65, c).

Origin from the ventral half of the inner surface of the mandibula for about one millimeter from the symphysis.

Insertion by muscle-fibres into the lateral half of the ventral surface of the body of the hyoid bone.

Relations.—Outer (ventral) surface with the stylohyoid (Fig. 65, d) and the mylohyoid (Fig. 65, c); medial border with the muscle of the opposite side. Lateral border with the hyoglossus (Fig. 67, h) and genioglossus (Fig. 67, f). Inner (dorsal) surface with the genioglossus.

Action.—Draws the hyoid forward.

M. jugulohyoideus (Fig. 67, b).—The jugulohyoid is a small flat quadrangular muscle covering the outer part of the ventral surface of the tympanic bulla.

Origin from the ventral border of the jugular process.

Insertion into the caudal side of the cartilaginous part of the stylohyoid opposite the origin of the styloglossus.

Relations.—Ventral surface with the digastric. Dorsal surface with the tympanic bulla (3).

Action.—Draws the hyoid backward and thus gives a firmer surface of origin for the styloglossus.

[114]

M. mylohyoideus (Fig. 65, c).—The mylohyoid is a large thin muscle seen after the reflection of the digastric, filling with its fellow of the opposite side the whole angle between the rami of the lower jaw. Its fibres are transverse.

Origin from the middle of the medial surface of the body of the mandible between the caudal opening of the mandibular canal and the symphysis of the jaw.

Fig. 67.—Muscles of Tongue, Hyoid Bone, and Pharynx.

a, M. tragicus lateralis; b, M. jugulohyoideus; c, M. pterygoideus externus; d, partially cut surface of M. pterygoideus internus; e, M. styloglossus; f, M. genioglossus; g, M. geniohyoideus; h, M. hyoglossus; i, M. glossopharyngeus; j, M. constrictor pharyngis medius; k, M. constrictor pharyngis inferior; l, M. stylopharyngeus; m, M. sternohyoideus (cut); n, M. cricothyreoideus; o, M. sternothyreoideus (cut); p, M. thyreohyoideus. 1, mandible; 1′, angular process of mandible; 2, stylomandibular ligament; 3, bulla tympani; 4, trachea; 5, œsophagus; 6, thyroid gland; 7, isthmus of the thyroid gland.

Insertion with the opposite muscle into a median raphe which extends from the symphysis of the jaw to the hyoid bone and is closely united to the external surface of the geniohyoid. Some of the fibres gain insertion into the body of the hyoid[115] bone through their insertion into the tendon of the stylohyoid (d).

Relations.—Outer surface with the submentalis (Fig. 64, c) and the digastric (Fig. 65, b). Lateral edge with the masseter (a), the digastric (b), and the mandibula. Caudal border with the stylohyoid (d). Inner (dorsal) surface with the geniohyoid (Fig. 67, g), the hyoglossus (Fig. 67, h), and the genioglossus (Fig. 67, f).

Action.—Raises the floor of the mouth and thus brings the hyoid forward.

M. ceratohyoideus.

Origin, the two proximal pieces of the cranial horn of the hyoid. The fibres pass caudad parallel to one another and within the middle constrictor of the pharynx.

Insertion into the whole length of the caudal cornu of the hyoid bone.

Relations.—Outer surface with the middle constrictor of the pharynx. Inner surface with the pharynx.

Action.—Draws craniad the body of the hyoid by bringing its cornua together and bending the joint between the two pieces of the cranial cornua.

The muscles connecting the hyoid and the tongue are described in connection with the description of the tongue.

III. MUSCLES OF THE BODY.

1. Muscles of the Back.

A. Muscles of the Shoulder

(connecting the forelimb with the back) (Fig. 68).

M. trapezius.—The trapezius muscle is divided in the cat into three portions, the spinotrapezius, the acromiotrapezius, and the clavotrapezius.

M. spinotrapezius, or trapezius inferior (Fig. 68, j).—The spinotrapezius (caudal part of the human trapezius) is a flat triangular muscle.

Origin from the tips of the spinous processes of all or nearly all the thoracic vertebræ and from the intervening supraspinous ligament. The origin may extend from a point anywhere between the first and fourth spines to any point between the eleventh and thirteenth.

[116]

Insertion along an S-shaped line (Fig. 76, k-l) which crosses the tuberosity of the scapular spine and forms an angle with the spine of about forty-five degrees. The line extends on one side onto the fascia covering the supraspinatus muscle, and on the other side onto the fascia covering the infraspinatus, but does not reach the borders of the scapula. The cranial two-thirds of the insertion is by a flat tendon which is broader craniad.

Relations.—Outer surface with the cutaneus maximus, and craniad with the acromiotrapezius (h). Inner surface with the latissimus dorsi (m) caudad and with the rhomboideus, supraspinatus, and infraspinatus craniad. Caudal border free; cranial border in contact with the acromiotrapezius (h).

Action.—Draws the scapula dorsocaudad.

M. acromiotrapezius, or trapezius superior (Fig. 68, h).—The acromiotrapezius (middle part of the human trapezius) is a flat four-sided muscle just craniad of the spinotrapezius, from the median dorsal line to the scapular spine.

Origin along the median dorsal line from the spinous process of the axis to a point anywhere between the spinous processes of the first and fourth thoracic vertebræ. Its caudal two-thirds is from a flat tendon which is directly continuous with the muscle of the opposite side and thus bridges the depression between the vertebral borders of the scapulæ. This tendon narrows craniad, and its cranial fourth is attached by a fascia to the spinous processes of the cervical vertebræ. At the cranial border of the muscle the fibres sometimes reach the middle line. The fibres of the muscle run nearly transversely.

Insertion (Fig. 76, h).—Into the outer surface of the metacromion, the glenoid border of the scapular spine from the metacromion to the tuberosity, and along a line continued for about one centimeter from the tuberosity onto the surface of the spinotrapezius (j) at the junction of its muscular and tendinous portions.

Fig. 68.—First Layer of Body-muscles, beneath the Skin and Skin-muscles.

a, M. frontoscutularis; b, M. masseter; c, M. sternomastoideus; d, M. clavotrapezius; e, M. clavobrachialis; f, M. levator scapulæ ventralis; g, M. acromiodeltoideus; h, M. acromiotrapezius; i, M. spinodeltoideus; j, M. spinotrapezius; k, caput longum of M. triceps brachii; l, caput laterale of M. triceps brachii; m, M. latissimus dorsi; n, M. xiphihumeralis; o, M. pectoralis minor; p, M. obliquus abdominis externus; q, M. sartorius; r, M. tensor fasciæ latæ; s, M. caudofemoralis; t, M. biceps femoris; u, M. semitendinosus; v, M. extensor caudæ lateralis; w, M. abductor caudæ externus; x, M. flexor caudæ longus; y, lumbodorsal fascia, superficial layer; z, fascia lata. 1, parotid gland; 2, submaxillary gland.

Relations.—Outer surface with the cutaneus maximus, the integument and platysma. Inner surface with a mass of fat partially covering the rhomboideus; with the occipitoscapularis, spinotrapezius, supraspinatus, levator scapulæ, and splenius.[117]
[118] Near its origin its caudal border touches the outer surface of the spinotrapezius (j). Its cranial border is overlaid near its insertion by the levator scapulæ ventralis (f) and is closely related to the caudal border of the clavotrapezius (d) near its origin.

Action.—The two muscles hold the scapulæ together.

M. clavotrapezius (Fig. 68, d).—The clavotrapezius (cranial part of the human trapezius) is a flat muscle passing from the lambdoidal crest and middorsal line to the clavicle and covering the side of the neck.

Origin.—The medial half of the lambdoidal crest and the middorsal line between the crest and the caudal end of the spine of the axis. The fibres are parallel and pass caudoventrad so that the muscle covers the ventral surface of the neck caudally and fills the concavity at the front of the shoulder.

Insertion into the clavicle and into a raphe between the clavotrapezius and the clavobrachial (e) muscles. This raphe extends along the clavicle and for some distance laterad of it. The muscle is partly continuous with the clavobrachial (e).

Relations.—Outer surface with the platysma, and at its craniomedial angle with the levator auris longus. Inner surface with the rhomboideus, splenius, occipitoscapularis, levator scapulæ, cleidomastoid, and supraspinatus. The cranial edge touches the sternomastoid. The caudal border touches the acromiotrapezius near the origin.

Action.—Draws the scapula craniodorsad.

This muscle and the clavobrachial (e) are sometimes considered as forming a single muscle, the cephalohumeral, or cephalobrachial.

M. occipitoscapularis (or levator scapulæ dorsalis or rhomboideus capitis) (Fig. 73, a).—The occipitoscapularis (equivalent to a part of the human rhomboideus) is a slender flat muscle from the lambdoidal ridge to the coracovertebral angle of the scapula.

Origin, the medial half of the lambdoidal ridge beneath the clavotrapezius (Fig. 68, d). The origin does not extend quite to the middle line. The muscle passes almost directly caudad. Its caudal end is thicker and narrower than the cranial end and[119] is wedged between the rhomboideus dorsally and the levator scapulæ ventrally.

Insertion either into the inner surface of the levator scapulæ near its dorsocranial border at its insertion or into the scapula near the coracovertebral angle (Fig. 78, f) or into both. Its fibres are very closely related to those of the rhomboideus.

Relations.—By its outer surface with the clavotrapezius (Fig. 68, d) and acromiotrapezius (h). At its insertion it is often overlaid by either the rhomboideus or the levator scapulæ or both. By its inner surface with the splenius (Fig. 73, b). Its caudal third is wedged between the rhomboideus and the levator scapulæ.

Action.—Draws forward and rotates the scapula.

M. rhomboideus (probably equivalent to the human rhomboideus major and rhomboideus minor).—A rather thick trapezoid muscle connecting the vertebral border of the scapula with the spinous processes of the adjacent vertebræ.

Origin from the caudal two or three fifths of the cervical supraspinous ligament and from the sides and tips of the first four thoracic vertebral spines and the interspinous ligaments caudad of these four spines. It passes lateroventrad, and the fibres converge somewhat so that the line of insertion is shorter than the line of origin.

Insertion by a short tendon (1 millimeter long) into the vertebral border of the scapula, the line of insertion passing gradually from the inner (Fig. 78, e) to the outer (Fig. 76, i) surface; and by fleshy fibres into the outer surface of the glenovertebral angle of the scapula (Fig. 76, i) in close connection with the origin of the teres major (j). A bundle of fibres at the cranial border may be inserted into the cranial border of the levator scapulæ at its insertion.

Relations.—By its outer surface with the spinotrapezius (Fig. 68, j), acromiotrapezius (h), latissimus dorsi (m), and clavotrapezius (d); it is usually partially separated from these muscles by a mass of fat. By its inner surface near its insertion with the serratus anterior (Fig. 73, i) and levator scapulæ (Fig. 73, h), and craniad with the splenius (Fig. 73, b), and the tendon of the serratus posterior superior (Fig. 73, l). At[120] its lateral end it touches the occipitoscapularis (Fig. 73, a), serratus anterior (i), levator scapulæ (h), supraspinatus (Fig. 75, a), and infraspinatus (Fig. 75, c), and the teres major (Fig. 75, d).

Action.—Draws the scapula toward the vertebral column.

M. cleidomastoideus (clavicular portion of the human sternocleidomastoid) (Fig. 65, h).—A narrow flat muscle between the mastoid process and the clavicle.

Origin from the apex and caudal margin of the mastoid process of the temporal bone, by tendon and muscle fibres. The muscle passes caudad, becoming broader and thinner.

Insertion into the lateral four-fifths of the clavicle within the insertion of the clavotrapezius, and into the raphe which continues laterad of the clavicle.

Relations.—Outer or lateral surface at the caudal end with the clavotrapezius (Fig. 65, j); further craniad with the depressor conchæ (Fig. 64, b), the platysma, and the sternomastoid (Fig. 65, g). Inner or medial surface with the supraspinatus (Fig. 75, a) caudad, with the mass of fat in the axilla at its middle, with the levator claviculæ ventralis (Fig. 65, i) and obliquus capitis superior (Fig. 71, e) craniad.

Action.—Pulls the clavicle craniad when the head is fixed. Turns the head and depresses the snout when the clavicle is fixed.

M. levator scapulæ ventralis, or levator claviculæ (Fig. 68, f; Fig. 65, i; Fig. 72, c, c′, c″) (not usually found in man).

Origin (Fig. 72, c′, c″) in two parts. (1) By fleshy fibres from the ventral surface of the atlantal transverse process near its caudal border (c′). (2) By a small tendon from the ventral surface of the basioccipital opposite the middle of the length of the bulla tympani (c″) in common with the longus capitis (a). The two heads unite to form a flat band which passes caudoventrad and is inserted (Fig. 76, f) by a flat short tendon, about eight millimeters broad, into the outer surface of the metacromion along its ventral border, and into the infraspinatus fossa for one or two millimeters distad of that border.

Relations.—Near the insertion the dorsal border overlaps the border of the acromiotrapezius (Fig. 68, h), and the ventral[121] border is attached by strong fascia to the border of the clavotrapezius (Fig. 68, d). The outer surface is in relation with the longissimus capitis (Fig. 73, g), clavotrapezius (Fig. 68, d), cleidomastoid (Fig. 73, d), platysma, and the skin. The inner surface is in relation caudad with the supraspinatus and with a mass of fat and a lymphatic gland which occupy the hollow of the shoulder. Craniad it is in relation with the cleidomastoid (Fig. 73, d), the scalenus (Fig. 73, f), the longus capitis (Fig. 73, e), and the obliquus superior (Fig. 71, e).

Action.—Pulls the scapula craniad.

M. latissimus dorsi (Fig. 68, m; Fig. 65, q; Fig. 77, e).—A large triangular sheet covering rather more than the dorsocranial half of the abdomen and thorax. It arises from the middorsal line, from the pelvis to the fifth thoracic spine, and is inserted into the humerus.

Origin from the tips of the neural spines of the vertebræ from the fourth or fifth thoracic to about the sixth lumbar. As far as the tenth or eleventh thoracic vertebra the origin is directly by muscle-fibres. Caudad of this is a broad triangular tendon which may be so closely united to the tendons of the underlying muscles that it cannot be separated. The muscle passes cranioventrad, the fibres converging to the axilla. In the axilla a part of the cutaneus maximus and the epitrochlearis (Fig. 65, r) take origin from the outer surface.

Insertion.—The muscle then ends in a flat tendon, to the cranial surface of which the fibres of the teres major (Fig. 77, c) are attached. Five to ten millimeters from the humerus the muscle-fibres of the teres give place to the tendon-fibres, and the conjoined tendon of the teres and latissimus which is thus formed (Fig. 79, d′) is inserted into a rough elongated area (Fig. 82, f) which lies parallel with the pectoral ridge on the medial surface of the shaft of the humerus (Fig. 48, g). The teres tendon forms the proximal part of the conjoined tendon. A part of the tendon of the latissimus may join the pectoralis minor at its distal or caudal border (Fig. 65), and thus it may contribute to the formation of both pillars of the bicipital arch (Fig. 65, t′).

[122]

Relations.—Its outer surface is covered caudad by the cutaneus maximus, and craniad by the spinotrapezius (Fig. 68, j) and the long head of the triceps (Fig. 68, k). The cranial border comes into relation with the spinotrapezius (j) and teres major (Fig. 77, c). The caudal border is in contact at the ventral end with the xiphihumeralis (Fig. 65, p) and pectoralis minor (Fig. 65, o). The inner surface of the latissimus covers near its cranioventral end parts of the xiphihumeralis (Fig. 68, n), pectoralis minor (Fig. 68, o), teres major (Fig. 77, c), and serratus anterior (Fig. 73, i). The caudal border covers a portion of the obliquus externus abdominis (Fig. 68, p); the dorsal and middle portions cover parts of the longissimus dorsi (Fig. 69, f″), the spinalis dorsi (Fig. 69, g), the serratus posterior superior (Fig. 73, l), and serratus posterior inferior (Fig. 73, n).

Action.—Pulls the arm caudodorsad.

M. serratus anterior and levator scapulæ (Fig. 73, i and h) in the cat are continuous at both origin and insertion, and it is barely possible to separate their contiguous borders without cutting the fibres. The muscle extends from the transverse processes of the last five cervical vertebræ and from the first nine or ten ribs to the medial surface of the scapula near its vertebral margin (Fig. 78, c and d).

M. serratus anterior (or serratus magnus) (Fig. 73, i).

Origin.—From the first nine or ten ribs in as many subdivisions. The first subdivision arises from the whole or nearly the whole of the caudal border of the first rib. The next four or five arise either from the ribs or from the costal cartilages near or at their junction with the ribs. The last four subdivisions arise from the ribs at increasing distances from the cartilages. The fibres converge and the subdivisions join one another.

Insertion (Fig. 78, c) into a narrow area on the medial surface of the scapula near the vertebral border. The glenoid half of the insertion is by a short tendon, the remainder by muscle-fibres.

Relations.—Outer (lateral) surface with the latissimus dorsi (Fig. 68, m), teres major (Fig. 77, c), subscapularis[123] (Fig. 77, a), and near the origin with the rectus abdominis (Fig. 73, k), the scalenus (Fig. 73, f-f‴), and the obliquus externus (Fig. 68, p). Inner (medial) surface with the serratus posterior superior (Fig. 73, l) and its tendon, and with the intercostales externi (Fig. 73, m). At its insertion the muscle touches the rhomboideus. The cranial edge is united to the caudal edge of the levator scapulæ (Fig. 73, h).

Action.—Depressor of the scapula.

M. levator scapulæ (Fig. 73, h).

Origin.—From the dorsal tubercles of the transverse processes of the last five cervical vertebræ and from the ligaments between the tubercles. A slip sometimes arises from the atlas.

Insertion (Fig. 78, d) into a triangular area on the medial surface of the scapula near its vertebral border. It is continuous with the insertion of the serratus anterior.

Relations.—Outer (lateral) surface with a mass of fat separating it from the clavotrapezius (Fig. 68, d) and cleidomastoid (Fig. 65, h), and with the subscapularis (Fig. 77, a). Inner (medial) surface with the splenius (Fig. 73, b), the tendon of the serratus posterior superior (Fig. 73, l), and the cervical portion of the longissimus dorsi (Fig. 69, f″). Near the insertion the muscle touches the occipitoscapularis (Fig. 73, a) and rhomboideus. Caudal border united with the serratus anterior (Fig. 73, i).

Action.—Draws the scapula cranioventrad.

B. Muscles of the Back

(interconnecting parts of the vertebral column) (Fig. 69).—The muscles connected with the vertebral column form a mass which is less markedly differentiated into distinct muscles than is the case in the limbs. A great longitudinal mass of fibres begins in the sacral region and extends along the vertebral column to the head. This mass contains fibres running in various directions and attached at one or both ends to the sacrum, the innominate bones, the spinous, transverse, and articular processes, and to the arches, of the vertebræ; to the ribs and to the head, and having in general the function of moving the vertebræ in various ways, or of moving parts connected with the vertebræ. In some regions, especially the cervical, it is possible to distinguish clearly distinct muscles[124] which have been differentiated from this mass. In the greater part of its extent, however, it is possible to distinguish only more or less closely interconnected bundles,—the more clearly marked of which have received special names.

The great dorsal mass running along each side of the vertebral column, occupying the space between the spinous processes and the transverse processes, receives the general name of M. extensor dorsi communis; the separate muscles are to be considered as parts or differentiations of this. The largest part of the mass is in the lumbar region (first portion of the longissimus dorsi, Fig. 69, f). At the caudal end of the thorax a lateral mass, parts of which become connected with the ribs, is separated off from the main part of the muscle; this extends craniad into the cervical region as M. iliocostalis (h). The main portion of the longissimus dorsi (f′) continues craniad on the medial side of the iliocostal into the neck region. At about the level of the eighth or ninth thoracic vertebra a strip begins to become separated off on the medial side of the longissimus; further forward it becomes clearly distinct, forming the spinalis dorsi (g). The separate muscles connecting the cervical vertebræ with each other and with the head are differentiations of the extensor dorsi communis.

A still deeper set of fibres, interconnecting the vertebræ, forms the multifidus spinæ and a number of other small muscles.

A description of the different bundles of fibres, under the names usually given, follows. Certain general principles are observed in some of the names used. Muscles which interconnect the spinous processes of the vertebræ receive the name spinalis. Those interconnecting contiguous spinous processes are called interspinalis. Muscles attached at one end to transverse processes, at the other to the spinous processes, receive the name transversospinalis. Semispinalis has the same signification as transversospinalis, but is a name usually applied to subdivisions of the transversospinalis group. The intertransversarii are muscles interconnecting the transverse processes.

[125]

Fig. 69.—Deep Muscles of Body, Exposed after Removal of the Muscles shown in Fig. 73.

a, M. biventer cervicis; b, M. complexus; c, M. obliquus capitis superior; d, M. longus atlantis; e, cut end of M. longissimus capitis; f, f′, f″, M. longissimus dorsi; g, M. spinalis dorsi; h, M. iliocostalis; i, Mm. intercostales externi; k, Mm. intercostales interni; l, M. transversus abdominis.

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(a) Muscles of the Lumbar and Thoracic Regions.

—The muscles of the lumbar and thoracic regions are mostly covered by a strong fascia, known as the lumbodorsal fascia (Fig. 68, y). This consists of two sheets, the superficial sheet being applied directly to the outer surface of the inner sheet, or separated from it by a mass of fat.

The superficial sheet (Fig. 68, y) overlies the lumbar region and the caudal half of the thoracic region. On the medial side it is attached to the spinous processes of the vertebræ and is united closely to the deeper layer. Laterally this fascia is continuous with the latissimus dorsi (m) and obliquus abdominis externus (p). Caudad it is attached to the spine of the ilium and becomes continuous with the fascia covering the gluteus muscles.

The deeper sheet (Fig. 70, c) is of a tendinous character, forming the external tendinous layer of the longissimus dorsi, many of whose fibres take origin from its under surface. It is described more fully in the account of this muscle.

M. longissimus dorsi (Fig. 69, f, f′, f″; Fig. 70, a and b).—A very large muscle, filling most of the region between the spinous processes and transverse processes of the lumbar and thoracic vertebræ, and extending into the cervical region.

The muscle is largest in the lumbar region (Fig. 69, f; Fig. 70), where it is divided into a narrow medial (Fig. 70, a) and a thick lateral portion (Fig. 70, b), the latter being again partially subdivided by the fascia. The two parts unite farther craniad.

The medial division (Fig. 70, a) consists of muscular bundles connecting the spinous processes of the vertebræ with the accessory and mammillary processes of other vertebræ; it is continuous caudad with the extensor caudæ lateralis (Fig. 70, f). The muscle-fibres take origin in the sacral region by small round tendons from the spinous processes of the last two sacral and the caudal vertebræ. They curve cranioventrad, forming a large belly, and are inserted into the accessory and mammillary processes of the lumbar vertebræ. In the thoracic region this division unites with that portion of the lateral division[127] in which the fibres coming from the fascia of origin are inserted into the laminæ and transverse processes of the thoracic vertebræ.

The lateral division (Fig. 70, b) is much larger than the medial one, forming in the lumbar region a nearly cylindrical mass.

Fig. 70.—Muscles on the Dorsal Side of the Vertebral Column in the Lumbar, Sacral, and Caudal Regions.

Both sheets of the lumbodorsal fascia have been removed, the deep layer (c) being cut where it passes into the longissimus dorsi. 1, crest of ilium; 4-7, tips of spinous processes of the fourth to seventh lumbar vertebræ. I, II, tips of spinous processes of first two sacral vertebræ. a, b, M. longissimi dorsi (a, medial portion; b, lateral portion; b′, portion taking origin from the lumbodorsal fascia); c, cut edge of deep layer of lumbodorsal fascia; d, M. multifidus spinæ; e, M. extensor caudæ medialis; f, M. extensor caudæ lateralis, g, M. abductor caudæ externus.

Origin from the crest of the ilium (Fig. 70, 1) and the medial surface of the ilium as far caudad as the auricular impression; also from the deep layer of the lumbodorsal fascia (c). This fascia is connected with the crest of the ilium and with the tips of the spinous processes of the vertebræ in the lumbar and thoracic regions, and from it a large proportion of the fibres of the longissimus take origin. In the lumbar region it dips into the muscle as an intermediate longitudinal sheet (c), partially dividing it lengthwise into two parts. Fibres taking origin from the lateral surface of this intermediate sheet curve cranioventrad, and are inserted on the transverse processes of the lumbar vertebræ. Fibres taking origin from the medial surface of the sheet pass mediocraniad and are inserted into the accessory processes and the surfaces of the vertebral arches. In the thoracic region (Fig. 69, f′)[128] most of the fibres are inserted in separate bundles by small tendons on the transverse processes of the vertebræ, while some of the medial fibres unite with tendinous strands which become attached to the laminæ and articular processes of the vertebræ. At the region of the eighth or ninth thoracic vertebra the spinalis dorsi (Fig. 69, g) begins to be separated off on the medial side, the separation becoming complete only some distance farther craniad; the longissimus dorsi then continues into the cervical region (f″). Bundles of fibres become attached in the manner above described to transverse processes of all the thoracic vertebræ. In the cervical region (f″) the muscle spreads out and becomes thinner, and bundles of fibres become attached to the transverse processes of the cervical vertebræ as far forward as the second. In the more cranial portion of its extent the muscle receives fibres having origin on the articular processes and laminæ of the cervical and a few of the more cranial thoracic vertebræ.

The portion of the muscle which is inserted on the cervical transverse processes (f″) is sometimes distinguished as the longissimus cervicis; it is not well separated from the rest of the muscle in the cat. The longissimus capitis (Fig. 69, c; Fig. 73, g) is to be considered a differentiated cranial portion of this muscle.

Relations.—Outer surface with the following, beginning with the caudal end: the sartorius (Fig. 68, q), the latissimus dorsi (Fig. 68, m), the lumbodorsal fascia (Fig. 68, y), the serratus posterior inferior (Fig. 73, n) and superior (Fig. 73, l), and the levator scapulæ (Fig. 73, h). Lateral margin with the abdominal muscles, the iliocostal (Fig. 69, h), and the levator scapulæ (Fig. 73, h). Medial side with the multifidus spinæ (Fig. 70, d), the spinous processes of the thoracic vertebræ, the spinalis dorsi (Fig. 69, g), the complexus (Fig. 69, b), the biventer cervicis (Fig. 69, a), and the longissimus capitis (Fig. 69, e).

Action.—Extends the vertebral column.

M. iliocostalis (Fig. 69, h).—This is a muscle about 1½ to 2 centimeters wide, lying on the dorsal portion of the ribs, laterad of the longissimus dorsi (f′). It is composed of many[129] partly separated bundles, each with an oblique tendon. The muscle begins at the last or next to the last rib, where it is connected with the longissimus by a rather small bundle of fibres. The rest of the muscle-fibres take origin in bundles from the lateral surface of the ribs, at about the angles, or from thin tendons connecting the angles, of the ribs; they pass obliquely craniad and are inserted, usually by tendons, on the lateral surface of the third or fourth rib craniad of the one on which the given bundle has origin. The insertions on any given rib lie ventrad of the origins on the same rib. In the cat this muscle is confined to the thoracic region.

Relations.—Outer surface with the serratus posterior inferior (Fig. 73, n) and superior (Fig. 73, l), and the levator scapulæ (Fig. 73, h). Inner surface with the dorsal surface of the ribs, the external intercostals (Fig. 69, i), and craniad with the levatores costarum. Medial edge in contact with the longissimus dorsi (Fig. 69, f′).

Action.—Draws the ribs together.

M. spinalis dorsi (Fig. 69, g).—A muscle interconnecting the spinous processes of vertebræ some distance apart, in the thoracic and cervical regions. It lies on the medial side of the longissimus dorsi (f′).

Origin by strong tendons from the tips of the spinous processes of the tenth to the thirteenth thoracic vertebræ. These tendons represent the cranial portion of the deep layer of the lumbodorsal fascia, and give origin also to many fibres of the longissimus dorsi, so that the two muscles are completely united at the origin of the spinalis. They become separated at about the level of the sixth thoracic vertebra, the spinalis forming a strong separate bundle passing into the neck region close against the sides of the spinous processes. The insertion is by fleshy bundles into the sides of the spinous processes of the first nine or ten thoracic vertebræ and of the cervical vertebræ as far forward as the second. Some of the fibres of this muscle pass craniad into the biventer cervicis (Fig. 69, a).

Relations.—Outer surface with the tendons of the serratus posterior inferior (Fig. 73, n) and serratus posterior superior (Fig. 73, l), and with the splenius (Fig. 73, b), biventer[130] cervicis (Fig. 69, a), and complexus (Fig. 69, b). Lateral and ventral surface with the longissimus dorsi (f′); medial surface with the spinous processes and the multifidus spinæ.

Action.—Extensor of the vertebral column.

M. multifidus spinæ.—This consists of bundles of fibres which have origin on the transverse processes or neighboring parts, pass craniodorsad across one or more vertebræ, and are inserted into the spinous processes of vertebræ lying some distance craniad of the origin. They lie deeper than the muscles previously described. The muscle is most strongly developed in the lumbar region (Fig. 70, d), where it forms a thick interwoven mass in which it is difficult to distinguish separate bundles. The fibres in this region have origin on the accessory or mammillary processes and usually pass over more than one vertebra between origin and insertion; their insertions reach the dorsal ends of the spinous processes, so that part of the muscle lies immediately beneath the lumbar fascia. In other regions the multifidus is covered by other muscles. In the thoracic region the separate bands are more distinct, and usually pass in their course over but one intervening vertebra. In the cervical region the bands are interconnected, forming a fairly distinct single muscle, which is described separately below as the semispinalis cervicis (Fig. 71, c). The portion of this muscle attached to the head (semispinalis capitis) forms the biventer cervicis (Fig. 69, a) and complexus (b). Caudad this muscle passes onto the tail as the extensor caudæ medialis (Fig. 70, c).

Relations.—Outer and lateral surface in the lumbar region with the longissimus dorsi (Fig. 69, f) and the lumbodorsal fascia (Fig. 68, y); in the thoracic region with the longissimus dorsi (Fig. 69, f′) and spinalis dorsi (g). Inner surface with the arches, articular processes, and spinous processes of the vertebræ.

Action.—Extends the back when the muscles of both sides work together. Turns the vertebral column obliquely sideways when one set acts alone.

The deepest layer of the multifidus forms what is sometimes[131] distinguished as the Mm. rotatores; no separate layer of this sort is to be made out in the cat.

Mm. interspinales.—Muscle-bundles passing from the spinous process of one vertebra to that of the vertebra immediately craniad or caudad of it. They are best developed in the lumbar region.

Mm. intertransversarii.—Muscle-fibres interconnecting the transverse processes. In the lumbar region the intertransversarii mediales connect the accessory and mammillary processes; the intertransversarii laterales lying between the transverse processes. In other regions only one set of the intertransversarii is to be distinguished.

(b) Dorsal Muscles of the Cervical Region.

—The clavotrapezius (Fig. 68, d) and occipitoscapularis (Fig. 73, a) have been described in connection with the muscles of the shoulder. The remaining muscles of this region may be considered as differentiations of the general vertebral musculature (M. extensor dorsi communis).

M. splenius (Fig. 73, b.)—A large sheet of muscle covering the dorsal part of the side of the neck, beneath the trapezii.

Origin from the whole cervical ligament and from the fascia covering the deeper muscles along a line which extends from the first thoracic spinous process caudolaterad for about two centimeters.

Insertion by a thin tendon into the whole lambdoidal ridge. Laterad the tendon may be fused with that of the longissimus capitis (Fig. 73, g).

Relations.—Outer surface with the sternomastoid (Fig. 68, c), occipitoscapularis (Fig. 73, a), clavotrapezius (Fig. 68, d), rhomboideus, tendon of the serratus posterior superior (Fig. 73, l), and the levator scapulæ (Fig. 73, h). Lateral edge closely united with the longissimus capitis (Fig. 73, g). Inner surface with the longissimus capitis, complexus (Fig. 69, b), and biventer cervicis (Fig. 69, a).

Action.—Lateral flexor of the head. The two together elevate the head.

M. longissimus capitis (trachelomastoideus) (Fig. 73, g; Fig. 69, e).—This is a slender muscle lying close against the[132] lateral border of the splenius (Fig. 73, b), and sometimes fused with the splenius at its craniad end. It is a cranial continuation of the longissimus dorsi (Fig. 69, f″).

Origin by five slips which are attached by strong tendons to the anterior articular processes of the last four cervical vertebræ. The tendons are common to this muscle and the complexus. The five slips unite to form a flat belly which has its

Insertion by a strong rounded tendon into the mastoid process of the temporal bone. The tendon may be closely united to that of the splenius.

Relations.—Lateral surface with the sternomastoid (Fig. 68, c), the levator scapulæ (Fig. 73, h), and longissimus dorsi (Fig. 69, f″). Medial surface with the splenius (Fig. 73, b) (to which it is partly united), the complexus (Fig. 69, b), and the longus atlantis (Fig. 69, d).

Action.—Lateral flexor of the head.

M. biventer cervicis (Fig. 69, a) (medial portion of M. semispinalis cervicis et capitis).—The biventer cervicis is a flat muscle which lies beneath the splenius (Fig. 73, b), next the median line of the neck, with its fibres longitudinal.

Origin in three or four slips from the surface of the three or four aponeurotic arches which take origin from the tips of the spinous processes of the last cervical and the first three thoracic vertebræ and pass laterad and caudad to the anterior processes of the second, third, fourth, and fifth thoracic vertebræ. These arches are deviated from their straight course by four aponeurotic bands which pass from them obliquely caudad and dorsad to the sides of the spinous processes of the vertebræ into which the arches are inserted. The arches form the beginning of the sheet of fascia which passes outside of the longitudinal supraspinous muscles, and the anchoring bands form the beginning of a similar sheet which passes within the longitudinal supraspinous muscles. An additional slip may take origin from the cervical ligament between the last cervical and the first thoracic vertebræ. The fibres form a flat band divided by two or three oblique tendinous intersections. It adheres closely to the cervical supraspinous ligament.

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Insertion by a strong tendon into the medial part of the lambdoidal crest.

Relations.—Outer surface with the splenius (Fig. 73, b) and at the caudal end with the longissimus dorsi (Fig. 69, f″). Inner surface with the spinalis dorsi (Fig. 69, g), the semispinalis cervicis (Fig. 71, c), and the rectus capitis posterior major (Fig. 71, a). Medial edge with the muscle of the opposite side; lateral edge with the complexus (Fig. 69, b).

Action.—Raises the snout.

M. complexus (Fig. 69, b) (lateral portion of M. semispinalis cervicis et capitis).—The complexus is closely associated with the biventer cervicis (a) and lies on its lateral side beneath the splenius (Fig. 73, b).

Origin by tendinous bands from the anterior articular processes of the last five or six cervical and the first one, two, or three thoracic vertebræ. The same bands give origin externally to the fibres of the longissimus capitis (Fig. 69, e), and internally to those of deeper muscles of the neck. The six or more muscular slips continued from these tendons unite to form a flat band, the

Insertion of which is by a flat tendon into the medial third of the lambdoidal crest some distance ventrad of the free edge of the crest. Near the insertion the lateral border of the muscle is connected by an aponeurotic band to the lateral border of the transverse process of the atlas.

Relations.—Outer surface at the dorsal border with the biventer cervicis (Fig. 69, a); at the middle with the splenius (Fig. 73, b); at the ventral border with the longissimus capitis (Fig. 69, e) and the longissimus dorsi (Fig. 69, f″). Inner surface with the cervical portion of the spinalis dorsi, with the semispinalis cervicis (Fig. 71, c), the obliquus capitis inferior (Fig. 71, b), the rectus capitis posterior major (Fig. 71, a), and the obliquus superior (Fig. 71, e).

Action.—Raises the head.

M. spinalis dorsi extends into the cervical region; it has been described.

M. semispinalis cervicis (Fig. 71, c).—This represents that portion of the multifidus spinæ which extends into the[134] cervical region. It is not composed of distinct bundles, like the multifidus of the thoracic region, but all the fibres are united into a fairly well-defined muscle.

Origin from the articular processes of the last five cervical vertebræ.

Insertion into the spinous processes of the cervical vertebræ up to the second, the largest part of the muscle being inserted into the caudal end of the spine of the atlas.

Relations.—Outer surface with the spinalis dorsi (Fig. 69, g) and the complexus (Fig. 69, b). Inner surface with the vertebræ.

Action.—Extends the neck.

M. longissimus dorsi extends into the cervical region, where it is often distinguished as the longissimus cervicis (Fig. 69, f″). It has been described.

M. longus atlantis (Fig. 71, f; Fig. 69, d).—This represents a differentiated portion of the longissimus dorsi.

Origin from the transverse process and the side of the vertebral arch of the third cervical vertebra.

Insertion into the caudolateral angle of the wing of the atlas.

Relations.—Dorsal surface with the complexus (Fig. 69, b), the longissimus capitis (Fig. 69, e), and the obliquus inferior (Fig. 71, b). Ventral surface with the scalenus (Fig. 71, g), longissimus dorsi (Fig. 69, f″), and levator scapulæ (Fig. 71, h).

Action.—Extends the neck and turns the head sideways.

M. rectus capitis posterior major (Fig. 71, a).

Origin.—The whole length of the spinous process (or crest) of the axis, or its caudal part only. It is united by a raphe to the opposite muscle. The muscle passes craniad and laterad to its

Insertion into an area ventrad of the medial part of the lambdoidal crest beneath the insertion of the complexus (Fig. 69, b) and the biventer cervicis (Fig. 69, a).

Relations.—Dorsal surface with the biventer cervicis (Fig. 69, a). Ventral surface with the rectus capitis posterior medius (Fig. 71, d) and the obliquus capitis inferior.

[135]

Action.—Raises the snout.

M. rectus capitis posterior medius (Fig. 71, d). (This is in man a part of the rectus capitis posterior major.)—It lies beneath the rectus capitis posterior major (Fig. 71, a).

Fig. 71.—Deep Muscles of the Neck, after the Removal of Mm. Biventer Cervicis and Complexus.

a, M. rectus capitis posterior major; b, M. obliquus capitis inferior; c, M. semispinalis cervicis; d, M. rectus capitis posterior medius; e, M. obliquus capitis superior; f, M. longus atlantis; g, M. scalenus (part of cervical portion); h, cut end of M. levator scapulæ.

Origin from the cranial end of the axial spine. It forms a triangular prismatic mass which passes craniad and has its

Insertion into an elongated area on the occipital bone, ventrad of the median half of the lambdoidal crest.

Relations.—Dorsal surface with the rectus capitis posterior major (Fig. 71, a). Ventral surface with the atlas and the rectus capitis posterior minor.

Action.—Assists the rectus capitis posterior major.

M. rectus capitis posterior minor.—A small triangular muscle beneath the rectus capitis posterior medius (Fig. 71, d).

Origin by the apex of the triangle from the cranial border of the dorsal arch of the atlas for about two millimeters laterad of the median line. The muscle passes over the ligament which connects the atlas to the occiput and adheres to it.

Insertion into an elongated area on the occipital bone,[136] ventrad of the insertion of the rectus capitis posterior medius for about five millimeters next to the median line.

Relations.—Dorsal surface with the rectus capitis posterior medius. Ventral surface with the posterior atlanto-occipital membrane.

Action.—Assists the rectus capitis posterior major.

M. obliquus inferior (Fig. 71, b).

Origin from the whole lateral surface of the spine of the axis. The parallel fibres form a thick mass which passes laterad and craniad.

Insertion into the dorsal surface of the transverse process of the atlas along a narrow area near its lateral margin.

Relations.—Outer surface with the rectus capitis posterior major (Fig. 71, a), the complexus (Fig. 69, b), and the obliquus superior (Fig. 71, e). Inner surface with the axis.

Action.—Rotates the head.

M. obliquus capitis superior (Fig. 71, e; Fig. 72, e).—This is a triangular muscle passing from the outer border of the atlas laterad of the recti capitis posteriores to the occiput.

Origin from the lateral border of the transverse process of the atlas on its ventral margin. The fibres pass craniad and diverge.

Insertion into the caudal side of the mastoid process of the temporal bone and into a line parallel with the lambdoidal ridge and ventrad of it, from the mastoid process to within one centimeter of the median line.

Relations.—Outer surface with the splenius (Fig. 73, b), complexus (Fig. 69, b), and longissimus capitis (Fig. 73, g). Inner surface with the obliquus inferior (Fig. 71, b), the wing of the atlas, and the rectus lateralis (Fig. 72, d). Ventral border with the cleidomastoid (Fig. 73, d) and one head of the levator scapulæ ventralis (Fig. 72, c′).

Action.—Flexes the head laterally.

Mm. interspinales and intertransversarii are found in the neck region also; they are described with the muscles of the back.

C. Muscles of the Tail.

—M. extensor caudæ medialis (Fig. 70, e).—This is a continuation caudad of the multifidus[137] spinæ (Fig. 70, d); it lies next to the dorsal median line, the muscles of right and left side touching one another in the middle line.

Origin by numerous fleshy bundles from the spinous processes of the sacral and first caudal vertebræ. The fibres pass caudad and are inserted by tendons into the articular processes and the dorsal surface of the caudal vertebræ.

Action.—Extends (raises) the tail.

M. extensor caudæ lateralis (Fig. 70, f).—This is a continuation caudad of the medial portion of the longissimus dorsi (Fig. 70, a); it lies just laterad of the extensor caudæ medialis (Fig. 70, e).

Origin in many fleshy bundles from the articular processes of the sacral vertebræ, and the transverse processes of the caudal vertebræ. The fibres curve dorsocaudad and are inserted by many long slender tendons on the dorsal surfaces of the caudal vertebræ. The muscle grows continually smaller as it passes caudad.

Action.—Raises the tail.

M. abductor caudæ (coccygis) externus (Fig. 68, w; Fig. 70, g).—A rounded muscle on the lateral surface of the cranial part of the tail.

Origin from the medial side of the dorsal border of the ilium, and from the dorsal surface of the sacrum. The muscle passes caudad, lying just ventrad of the extensor lateralis, and is inserted into the transverse processes and lateral surfaces of the caudal vertebræ, as far back as the eighth or ninth.

Action.—Bends the tail sideways.

M. abductor caudæ internus (or M. coccygeus).—A large flat muscle, having origin on the spine of the ischium. The muscle passes dorsomediad, spreading out, and is inserted into the transverse processes of the second to the fourth caudal vertebræ.

Relations.—Lateral surface with the caudofemoralis (Fig. 68, s), gluteus maximus, and pyriformis. Medial surface with the iliocaudalis and the flexor caudæ longus (Fig. 68, x).

M. iliocaudalis (Str.-D.) (Fig. 162, 11′, page 398).—This represents a portion of the levator ani of man, and in some[138] specimens is united with that muscle (Fig. 162, 11) in the cat.

Origin along the ventral half of the medial surface of the ilium, caudad of the sacrum. The fibres pass caudad and are inserted by a flat tendon into the ventral surface of the caudal vertebræ, from the second or third to about the seventh.

Relations.—Lateral surface with the gluteus maximus and abductor caudæ internus (Fig. 68, w). Medial surface with the levator ani (Fig. 162, 11), with which this muscle may be partly united.

Action.—The two muscles together flex the tail.

M. flexor caudæ longus (Fig. 162, 12; Fig. 113, c, page 270).

Origin on the ventral surface of the last lumbar vertebra, of the sacrum, and of the transverse processes of the caudal vertebræ. Caudad the muscle forms long, strong tendons which are inserted on the ventral surface of the tail.

Relations.—Lateral surface with the ilium, the gluteus maximus, the abductor caudæ internus, the iliocaudalis, and the skin of the tail. Dorsally it touches in the caudal region the abductor caudæ externus (Fig. 68, w), medially the flexor caudæ brevis (Fig. 162, 13).

Action.—Flexes the tail.

M. flexor caudæ brevis (Fig. 162, 13).

Origin on the ventral surface of the caudal vertebræ, from the first to the seventh or eighth. The principal head comes from the first vertebra. The bundles thus formed pass caudad and are inserted each into the ventral surface of a vertebra some distance caudad of the origin. The muscle extends to about the tenth caudal vertebra, and its most caudal parts are closely interconnected with the flexor caudæ longus.

Relations.—Lateral edge with the flexor caudæ longus. Medial edge in the sacral region with the levator ani (Fig. 162, 11) and the iliocaudalis (Fig. 162, 11′); in the caudal region with the muscle of the opposite side.

Action.—Flexes the tail.

2. Muscles on the Ventral Side of the Vertebral Column.

A. Lumbar and Thoracic Regions.

—The iliopsoas[139] (Fig. 162, 8) belongs in this group, but since it moves the leg it is described with the muscles of the pelvic limbs.

M. psoas minor (Fig. 162, 9, page 398).—This muscle lies along the ventral surface of the vertebral column, next to the peritoneum, and extends from the thoracic vertebræ to the ilium.

Origin usually by five heads, from the caudal border of the centra of the last two (or one) thoracic and first three (or four) lumbar vertebræ. These heads also serve as origin for a part of the iliopsoas, and are closely united at their origin with the quadratus lumborum. They unite to form an, at first, rather large flat muscle, which rapidly becomes smaller caudad, finally forming a slender tendon which is inserted on the iliopectineal line, just craniad of the acetabulum.

Relations.—Dorsolateral surface with the quadratus lumborum and iliopsoas (Fig. 162, 8). Ventral surface with the pleura and diaphragm near the origin; with the peritoneum caudad.

Action.—Flexes the back in the lumbar region.

M. quadratus lumborum.—A flat muscle lying on the ventral surface of the transverse processes of the lumbar vertebræ. Craniad it has origin on the ventral surface of the last two thoracic vertebræ, and by a few fibres from the last rib. The muscle passes caudad, lying on the lateral side of the psoas minor (Fig. 162, 9), and becomes attached to each of the transverse processes of the lumbar vertebræ. Caudad the muscle passes into a strong flat tendon which is inserted into the anterior inferior spine of the ilium.

Relations.—Dorsal surface with the transverse processes and the intertransverse muscles. Ventral and medial surfaces with the psoas minor (Fig. 162, 9) and iliopsoas (Fig. 162, 8).

Action.—Bends the vertebral column sideways.

B. Muscles on the Ventral Side of the Neck

(Fig. 65, page 109).—M. sternomastoideus (Fig. 65, g; Fig. 68, c).—The sternomastoid (sternal portion of the human sternocleidomastoid) is a flat band one to three centimeters wide extending from the cranial end of the manubrium and the midventral line craniad of it to the lambdoidal ridge.

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The origin extends from the cranial end of the manubrium along the midventral line as far as the caudal border of the cricoid cartilage, and is in two parts. The caudal portion arises from the lateral surface of the manubrium along the dorsal half of its cranial end, and from the median raphe for about one centimeter craniad of the manubrium. Its fibres are parallel and tend to diverge into two layers. The cranial portion arises by means of fibres which cover the median line and interdigitate with the fibres of the opposite muscle between the cricoid cartilage and a point a few millimeters craniad of the manubrium. Its caudal border is thus overlaid by the caudal portion. Its fibres converge and join those of the caudal portion. The muscle passes dorsocraniad and is

Inserted by means of a flat tendon into the lateral half of the lambdoidal ridge and into a continuation of the ridge onto the mastoid portion of the temporal bone as far as the mastoid process. The thickest part of the tendon is inserted into the mastoid process.

Relations.—Outer surface at the caudal end with the pectoralis major (Fig. 65, l); in the middle part with the platysma, the depressor conchæ (Fig. 64, b), and with the external jugular vein (Fig. 65, 5), which crosses it obliquely; at the cranial end with the submaxillary (Fig. 65, 2) and parotid (1) glands. Inner surface with the sternohyoid (Fig. 65, e), sternothyroid (g′), internal jugular vein, longus capitis muscle, the cleidomastoid (h), levator scapulæ ventralis (i), a large lymphatic gland ventrad of the ear, and the splenius (Fig. 73, b).

Action.—One muscle turns the head and depresses the snout. Both together depress the snout.

M. sternohyoideus (Fig. 65, e).—A slender muscle on the midventral line of the neck close to the opposite muscle.

Origin from the cranial border of the first costal cartilage. The muscle passes craniad closely united near its caudal end to the sternothyroid (g′).

Insertion (Fig. 67, m) into the outer half of the ventral surface of the body of the hyoid bone caudad of the origin of the geniohyoid (Fig. 67, g).

Relations.—Outer surface with the sternomastoid[141] (Fig. 65, g). Inner surface with the thyrohyoid (Fig. 65, f; Fig. 67, p), the trachea (Fig. 67, 4), and the larynx.

Action.—Draws the hyoid caudad. Raises the ribs and sternum when the hyoid is fixed.

M. sternothyreoideus (Fig. 65, g′).—The sternothyroid lies beneath the sternohyoid and is connected with it at its caudal end.

Origin from the first costal cartilage beneath the sternohyoid.

Insertion (Fig. 67, o) into the lateral part of the caudal border of the thyroid cartilage of the larynx.

Relations.—Outer surface with the sternohyoid (Fig. 65, e) and laterally with the sternomastoid (Fig. 65, g). Inner surface with the trachea (Fig. 67, 4), and at the lateral edge with the thyroid gland (Fig. 67, 6).

Action.—Pulls the larynx caudad.

M. scalenus (Fig. 73, f-f‴).—This is a large and complex muscle lying on the ventral side of the neck and the lateral surface of the thorax. It might equally well be classified with the muscles of the thorax. It is divisible into a considerable number of interconnected bundles which are sometimes described as separate muscles.

The largest, middle portion (scalenus medius) (f′) takes origin by thin tendons from the sixth, seventh, eighth, and ninth ribs, just dorsad of their junction with the cartilages; these tendons unite near their origins to form a flat band which becomes fleshy at about the fifth rib, passes craniad, and unites just craniad of the first rib with the other parts of the muscle. The dorsal portion (scalenus posterior) (f″) has origin by a very slender tendon from about the middle of the outer surface of the third or fourth rib; it extends craniad as a narrow band which passes laterad between the heads of the serratus anterior (i) that are attached to the second and third (or third and fourth) ribs, and unites with the other heads. The ventral head (part of scalenus anterior) (f‴) arises by one or two minute tendons from the cartilages of the second and third ribs; they are partly united with the transversus costarum (j). This ventral head passes craniad and unites with the other heads.

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These three divisions unite just craniad of the first rib with each other and with the cervical portion of the muscle (f) (part of scalenus anterior). This consists of a number of small bundles of fibres which arise from the first rib and the transverse processes of the first thoracic and the last six cervical vertebræ.

The insertion is onto the transverse processes of all the cervical vertebræ, including the axis and atlas. According to Strauss-Durckheim the fibres from any given transverse process are inserted into the transverse processes of all the vertebræ craniad of it.

The muscle is partly continuous craniad with the longus capitis (e). The cervical nerves pass out between the bundles of the cervical portion.

Relations.—Outer (lateral) surface in the thoracic region with the external oblique (Fig. 68, p), the pectoralis minor (Fig. 65, o), and in part with the serratus anterior (Fig. 73, i); in the cervical region with the sternomastoid (Fig. 65, g). Inner (medial) surface with the serratus anterior (Fig. 73, i), the levator scapulæ (Fig. 73, h), the longus atlantis, the transverse processes of the cervical vertebræ, and the longus capitis (Fig. 73, e).

Action.—Flexes the neck and draws the ribs craniad.

M. longus capitis (or rectus capitis anterior major) (Fig. 72, a; Fig. 73, e).—This is a long muscle lying on the ventral aspect of the cervical vertebræ. With the muscle of the opposite side and the underlying longus colli (Fig. 72, g′) it forms a trough in which lie the œsophagus, pharynx, and trachea.

Origin by five (or six) heads from the ventral margins of the transverse processes of the cervical vertebræ from the second to the sixth inclusive. The heads unite into a common belly, the lateral border of which is united with the levator scapulæ ventralis (Fig. 72, c) near its insertion.

Insertion into the body of the sphenoid bone between the bulla and the middle line. The insertion extends craniad onto the basisphenoid.

Relations.—Ventral surface with the sternomastoid (Fig. 65, g) and the large lymphatic gland ventrad of the external[143] ear. Lateral surface with the scalenus (Fig. 72, h) and craniad with the levator scapulæ ventralis (Fig. 72, c-c″). Medial surface with the carotid artery, the trachea, œsophagus, and pharynx. Dorsal edge with the vertebræ, the longus colli (Fig. 72, g′), and the rectus capitis anterior minor (Fig. 72, b).

Action.—Lowers the snout.

M. rectus capitis anterior minor (Fig. 72, b).—This is a flat band beneath (dorsad of) the longus capitis (a).

Origin from the ventral surface of the inferior arch (body) of the atlas (3) for about five millimeters laterad of the middle line.

Insertion into a deep depression on the basioccipital caudad of the insertion of the longus capitis (a) and between the caudal end of the bulla (1) and the median line.

Relations.—Ventral surface with the longus capitis (a). Dorsal surface with the anterior (ventral) atlanto-occipital membrane (2).

Action.—Depresses the snout.

Fig. 72.—Muscles on the Ventral Surface of the Cervical Vertebræ.

On the left side the longus capitis and one head of the levator scapulæ ventralis have been cut, in order to show the rectus capitis anterior minor. a, M. longus capitis; b, M. rectus capitis anterior minor; c, M. levator scapulæ ventralis (c′, its atlantal head; c″, its occipital head); d, M. rectus capitis lateralis; e, M. obliquus capitis superior; f, M. longissimus capitis; g, M. longus colli (g′, its cervical portion; g″, its thoracic portion); h, part of M. scalenus. 1, bulla tympani; 2, ventral atlanto-occipital membrane; 3, ventral arch of atlas; 4, first rib.

M. rectus capitis lateralis (Fig. 72, d).—This lies ventrad of the obliquus superior (e) on the ventral face of the transverse process of the atlas and at first appears to be a part of the obliquus superior.

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Origin from the median half of the ventral surface of the transverse process of the atlas. The fibres form a cylindrical mass which passes craniad and slightly laterad.

Insertion into the fossa laterad of the condyle of the occipital bone.

Relations.—Ventral surface with the digastric (Fig. 65, b) and the lymphatic gland ventrad of the ear. Dorsal surface with the wing of the atlas.

Action.—Flexes the head laterally.

M. longus colli (Fig. 65, g′, g″).—A long, rather slender muscle lying on the ventral surface of the cervical and first six thoracic vertebræ. It consists of many separate bundles, and is divisible into a thoracic and a cervical portion.

The thoracic portion (g″) has origin by six heads from the ventral surface of the first six thoracic vertebræ. The separate heads join to form a band which passes craniad and is inserted for the most part into the processus costarius of the sixth cervical vertebra; a portion of it joins the cervical division of the muscle.

The cervical portion (g′) is a continuation craniad of the thoracic portion. It arises in small bundles from the transverse processes and sides of the ventral surfaces of the centra of the cervical vertebræ. These bundles pass craniomediad, so that those from the muscles of opposite sides meet and are inserted on the centra of the vertebræ in the middle line, each pair of bundles forming a V opening caudad. The most cranial insertion is into the tubercle on the middle of the ventral surface of the ventral arch of the atlas (3).

Relations.—Dorsal surface with the centra of the vertebræ. Ventral surface in the thorax with the trachea; in the neck with the œsophagus. Medial edge with the muscle of the opposite side. Lateral margin in the cervical region with the longus capitis (a).

Action.—Bends the neck.

3. Muscles of the Thorax.

A. Breast-muscles (connecting the arm and thorax)

(Fig. 65, page 109).

The muscles connecting the arm with the sternum, corresponding to the pectoralis muscles of man, fall into a considerable[145] number of not always clearly separated bundles in the cat. We shall distinguish by name four such bundles; these might be, and frequently are, further subdivided.

M. pectoantibrachialis (Fig. 65, m).—This is a small flat bundle about one or two centimeters wide, forming the most superficial portion of the pectoralis group.

Origin on the lateral surface of the manubrium. The muscle passes laterad onto the arm and is inserted by a flat tendon into the superficial fascia of the dorsal border of the forearm, near the elbow-joint. The tendon is continuous craniad with the border of the clavobrachial (k), so that a bundle of fibres may pass from this muscle to be inserted with the clavobrachial, or a bundle from the clavobrachial to be inserted with this. The tendon is continuous caudad with that of the epitrochlearis (r). The fascia of insertion may be traced to an attachment into the dorsal border of the ulna. This muscle usually receives near its insertion a slip, sometimes called its caudal division, from the deep layer of the pectoralis major.

Relations.—Outer surface with the integument. Inner surface with both divisions of the pectoralis major (l) on its medial two-thirds, and with the biceps (t) and bicipital arch (t′) in its lateral one-third. The cranial border is closely applied to that of the clavobrachial (k), except at the two ends.

Action.—Draws the arm mediad.

M. pectoralis major, or ectopectoralis (Fig. 65, l, l′).—In this it is usually possible to distinguish a superficial and a deep layer.

The superficial (l′) layer is a flat band of fibres one or two centimeters wide.

Origin from a raphe in the midventral line, along the cranial half of the manubrium and for five to ten millimeters craniad of it.

Insertion (Fig. 81, f).—The pectoral ridge of the middle third of the shaft of the humerus, slightly dorsad of the line of insertion of the deep portion.

Relations.—Outer surface with the clavobrachial (Fig. 65, k) and integument, the medial end caudally with the pectoantibrachialis (m), and the distal end with the brachialis[146] (v). Inner surface with the deep portion (l), the fibres of which it crosses obliquely.

The deep portion (l) is a flat band of parallel fibres about three times as broad as the superficial portion.

Origin.—The ventral surface of the manubrium and of the first three divisions of the sternum, and the median raphe for about one centimeter craniad of the manubrium. It passes directly laterad.

Insertion (Fig. 81, e) along a line which begins at the infraspinatus fossa of the great tuberosity, and runs parallel to the deltoid ridge until it reaches the pectoral ridge at the junction of the second and third fourths of the humerus and then continues in the direction of the pectoral ridge as far as the junction of the third and fourth fourths of the humerus. Some of its fibres may pass caudad of the biceps and be inserted with the epitrochlearis. Into the part of the line which is parallel to the deltoid ridge the muscle is inserted by a flat tendon; into the remainder of the line it is usually inserted directly by muscle-fibres, though at its caudal end its insertion may again be tendinous.

Relations.—Outer surface with the cutaneus maximus, the outer layer of the pectoralis major (Fig. 65, l′), the pectoantibrachialis (m), and the clavobrachial (k). Inner surface with the pectoralis minor (o), the proximal end of the humerus, the coracobrachialis (Fig. 77, f), and the supraspinatus (Fig. 77, d) at the insertion of the latter. At the cranial part of its origin it touches the sternomastoid (Fig. 65, g) and sternohyoid (Fig. 65, e).

Action.—Draws the arm mediad and turns the foot forward.

M. pectoralis minor, or entopectoralis (Fig. 65, o).—This is a fan-shaped mass of fibres, flat but thicker than the pectoralis major.

Origin from the lateral half of the first six divisions of the body of the sternum and sometimes from the xiphoid process. The fibres are divisible into several bundles which vary in extent and distinctness and are sometimes described as separate muscles. They pass craniolaterad and converge so that the line of insertion is about one-half as long as that of the origin.

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Insertion (Fig. 81, d) into the humerus along a line which forms the ventral border of the bicipital groove at the proximal end of the bone, passes thence onto the pectoral ridge and continues in the direction of the pectoral ridge nearly as far as the middle of the length of the humerus. The cranial and caudal fibres are inserted by means of a thin tendon; the middle fibres directly. The cranial border of the tendon of insertion is continued as a thin tendon to the coracoid process; it is closely united with the insertion of the supraspinatus.

Relations.—By its outer surface with the pectoralis major (Fig. 65, l) over its cranial half; with the cutaneus maximus, latissimus dorsi (Fig. 65, q), and integument over its caudal half. By its inner surface with the xiphihumeralis (Fig. 65, p), rectus abdominis (Fig. 73, k), and a mass of fat in the axilla. The tendon of the xiphihumeralis is also connected with its inner surface near its insertion. Its cranial border is free. Its caudal border is in relation at its medial end with the xiphihumeralis, at its lateral end with the tendon of the latissimus dorsi (Fig. 65, q).

Action.—Draws the arm toward the middle line.

M. xiphihumeralis (Fig. 65, q).—A long, thin, narrow muscle which may be considered as part of the pectoralis minor.

Origin a median raphe along the xiphoid process or at an angle to the median line on the rectus abdominis muscle. It passes craniad, becoming gradually smaller, and about two centimeters from its insertion ends in a thin tendon.

Insertion.—It ends in a flat tendon which in passing is connected by a strong fascia with the tendon of the latissimus (Fig. 65, q). It passes along the inner surface of the pectoralis minor (o) to be inserted with its cranial fibres near the ventral border of the bicipital groove.

Relations.—Outer surface with the cutaneus maximus at its medial end, with the pectoralis minor (o) at its distal end. Inner surface with the external oblique (Fig. 68, p), the rectus abdominis (Fig. 73, k), and a mass of fat in the axilla. Borders free. Some of the fibres of the cutaneus maximus are attached to its outer surface.

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Action.—Assists the pectoralis minor.

B. Muscles of the Wall of the Thorax.

—M. serratus posterior superior (Fig. 73, l).—A thin sheet of muscle and tendon beneath the serratus anterior (i) on the dorsal part of the thorax and neck.

Origin by fleshy slips from the outer surfaces of the first nine ribs just ventrad of their angles. The origin may extend as far as the tenth or eleventh ribs. The first slip is usually delicate. The fibres are directed dorsad and craniad and unite to form a continuous sheet. This ends along a longitudinal line opposite the vertebral transverse processes in the aponeurosis which covers the longissimus dorsi and other muscles in this region. Its insertion is into the median dorsal raphe between the axial spinous process and the tenth thoracic spinous process.

Relations.—Outer surface with the serratus anterior (i), the latissimus dorsi (Fig. 68, m), and the rhomboideus. Inner surface with the external intercostals (Fig. 73, m), the iliocostal (Fig. 69, h), the longissimus dorsi (Fig. 69, f), the spinalis dorsi (Fig. 69, g), and the splenius (Fig. 73, b).

Action.—Draws the ribs craniad.

M. serratus posterior inferior (Fig. 73, n).—The serratus posterior inferior is a thin muscle lying caudad of the serratus posterior superior (l) and sometimes overlying the caudal end of the latter.

Origin by four or five heads from the last four or five ribs. In some specimens only three heads may be present. The separate bundles pass dorsocaudad and unite to form a continuous sheet which ends in a continuation of the aponeurosis of the serratus posterior superior (l).

Insertion into the lumbar spinous processes and the intervening interspinous ligaments.

Relations.—Outer surface with the latissimus dorsi (Fig. 68, p), and at the origins with the intercostales externi (Fig. 73, m). Inner surface with the intercostales interni (Fig. 69, k) at the origin and with the intercostales externi (Fig. 69, i), iliocostal (Fig. 69, h), longissimus dorsi (Fig. 69, f), and the caudal end of the spinalis dorsi (Fig. 69, g).

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Fig. 73.—Body Muscles exposed after Removal of the Forelimb and the Muscles shown in Fig. 68.

a, M. occipitoscapularis (cut); b, M. splenius; c, M. levator scapulæ ventralis (cut); d, M. cleidomastoideus (cut); e, M. longus capitis; f, M. scalenus (f′, middle division; f″, dorsal division; f‴, ventral division; f, cervical portion); g, M. longissimus capitis; h, M. levator scapulæ (cut); i, M. serratus anterior (cut); j, M. transversus costarum; k, M. rectus abdominis (k′, its tendon); l, M. serratus posterior superior; m, M. intercostales externi; n, M. serratus posterior inferior; o, M. obliquus abdominis internus; p, lumbodorsal fascia.

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Action.—Assists the serratus posterior superior.

M. transversus costarum, or sternocostalis externus (Fig. 73, j).—This is a thin muscle applied to the cranial part of the side of the thorax, covering the cranial end of the rectus abdominis (k).

Origin by tendon from the side of the sternum between the attachments of the third and sixth ribs. The muscle passes dorsocraniad to its

Insertion on the first rib and the lateral portion of its costal cartilage.

Relations.—Outer surface with the pectoralis minor (Fig. 65, o). Inner surface with the scalenus (Fig. 73, f‴), the rectus abdominis (Fig. 73, k), and the intercostales externi (Fig. 73, m). At its insertion it is united with a portion of the scalenus (Fig. 73, f‴).

Action.—Draws the sternum forward.

Mm. levatores costarum.—Small muscles having origin on the transverse processes of the thoracic vertebræ, passing caudoventrad, and becoming inserted on the angle of the rib lying immediately caudad of the origin. They are continuous with the external intercostals.

Relations.—Outer surface with the longissimus dorsi (Fig. 69, f), and craniad with the iliocostal (Fig. 69, h). Inner surface with the internal intercostals (Fig. 69, k).

Action.—Pull the ribs dorsocraniad.

Mm. intercostales externi (Fig. 73, m; Fig. 69, i).—The external intercostals are placed in the outer portion of the intercostal spaces. They are composed of bundles of fibres attached by their ends to the adjacent borders of the ribs and having in general the direction of the external oblique muscle, i.e., they pass from their cranial ends caudoventrad. They occupy the intercostal spaces between the true ribs and extend even caudad into the spaces between the false ribs. They are lacking between the ventral ends of the costal cartilages of the first six to eight ribs, so that the internal intercostals (Fig. 69, k) are here exposed. The more caudal external intercostals are more nearly craniocaudal in direction.

Relations.—Outer surface with obliquus abdominis externus[151] (Fig. 68, p), latissimus dorsi (Fig. 68, m), serratus posterior inferior (Fig. 73, n) and superior (Fig. 73, l), serratus anterior (Fig. 73, i), scalenus (Fig. 73, f), and iliocostal (Fig. 69, h). Inner surface with the internal intercostals (Fig. 69, k).

Action.—Protractors of the ribs.

Mm. intercostales interni (Fig. 69, k).—The internal intercostals are similar to the external intercostals, beneath which they lie. Their fibres pass between the ribs at nearly right angles to those of the external intercostals and have nearly the direction of the fibres of the internal oblique. They occupy all the intercostal spaces from the first to the thirteenth ribs.

Relations.—Outer surface with the external intercostals (Fig. 69, i), and ventrad with the scalenus (Fig. 73, f), transversus costarum (Fig. 73, j), and rectus abdominis (Fig. 73, k). Inner surface with the pleura and the transversus thoracis.

Action.—Retractors of the ribs.

M. transversus thoracis (triangularis sterni; sternocostalis internus).—This represents a thoracic portion of the transversus abdominis. It consists of five or six flat muscular bands lying on the inner surface of the thoracic wall.

Origin on the lateral borders of the dorsal face of the sternum, opposite the attachments of the cartilages of the third to the eighth ribs. The six bands thus formed, each about one centimeter wide, pass laterad and are inserted into the cartilages of the ribs near their junction with the ribs, and into the fascia which covers the inner surface of the internal intercostals in this region.

Relations.—Outer surface with the internal intercostals and the cartilages of the ribs. Inner surface with the pleura.

Diaphragma (Fig. 74).—The diaphragm consists of a central so-called semilunar tendon (e) and of muscular fibres which pass radially from the body wall to the tendon. It forms a complete oblique partition between the abdominal and thoracic cavities. The dorsal end is farther caudad than the ventral.

The central tendon (e) is thin and irregularly crescent-shaped, with the convexity ventrad and the horns of the crescent prolonged as two tendinous bands (e′) which end in two[152] triangular membranous portions (d) of the diaphragm, one on each side of the spinal column. It is pierced by an opening for the vena cava (f).

Fig. 74.—Diaphragm, Caudal Surface seen Obliquely from the Right.

a, right crus; b, left crus; c, c′, sternocostal part of diaphragm; d, d′, membranous portions of the diaphragm; e, central tendon; e′, prolongations of central tendon; f, opening for posterior vena cava; g, œsophagus; h, aorta; i, M. transversus abdominis.

The muscular portion is in two parts: (a) The vertebral portion (a, b) arises by a single tendon from the ventral surface of the second, third, and fourth lumbar centra. The tendon diverges into two, the right one (a) of which is much stronger, and from each of the two arise muscle-fibres. Each mass of fibres is one of the two crura (sing. crus) of the diaphragm. The aorta (h) enters the abdomen between the crura dorsally. The fibres of each crus diverge to be inserted into the central tendon and the dorsal continuation of its horn. The two sets of fibres unite ventrad of the opening of the aorta. Between this opening and the central tendon is another opening for the œsophagus[153] (g); this lies entirely in that part of the diaphragm which arises from the right crus (a). The fibres are again united ventrad of the œsophageal opening.

(b) The sternocostal part (c, c′) takes origin from the xiphoid process and the last five ribs, by fleshy bundles which interdigitate with those of the transversus abdominis (i). The fibres converge to the central tendon (e). Between the crus of each side and the most dorsal of the costal fibres is the membranous interval (d) mentioned above.

4. Abdominal Muscles.

M. obliquus abdominis externus (Fig. 68, p).—A large, thin sheet of muscle covering the whole abdomen and part of the thorax ventrally.

Origin.—(a) From the last nine or ten ribs by means of as many tendons, which are interconnected to form arches that span the slips of the serratus anterior. The muscle-fibres arise from these tendons and from their intervening arches. (b) From the lumbodorsal aponeurosis common to it and the internal oblique. The cranial fibres pass nearly ventrad, the caudal fibres caudad, and the intervening fibres take an intermediate course. The fibres end in a thin aponeurosis of insertion along a curved line which passes at first caudad and then laterodorsad. The aponeurosis fibres continue in the direction of the muscle-fibres to the

Insertion into the median raphe ventrad of the sternum from the insertion of the seventh costal cartilage to the xiphoid process, into the linea alba from the sternum to the pubic tubercle, and into the tubercle and the cranial border of the pubis. Caudad of the xiphoid process the aponeurosis is closely united to the superficial layer of the internal oblique, where it forms the outer layer of the sheath of the rectus abdominis. Laterad of the pubic tubercle the tendon is perforated by the inguinal canal. In the cat neither the caudal part of the muscle nor its tendon is attached to the ilium, as it is in man and the dog, so that no Poupart’s ligament, or inguinal ligament, is formed.

Relations.—Outer surface with the cutaneus maximus, the integument, and near the origin with the latissimus dorsi (Fig. 68, m). Inner surface with the obliquus internus[154] (Fig. 73, o), the rectus abdominis (Fig. 73, k), the intercostales externi (Fig. 73, m), a small part of the serratus posterior inferior (Fig. 73, n), and by its dorsal tendon with the longissimus dorsi (Fig. 69, f).

Action.—Constrictor of the abdomen.

M. obliquus abdominis internus (Fig. 73, o).—A thin sheet similar to the preceding but of less extent. Its fibres cross those of the external oblique nearly at right angles and lie beneath them.

Origin.—(1) Between the fourth and seventh lumbar vertebræ from the lumbar aponeurosis which is common to it and the external oblique. The lumbar aponeurosis takes origin from the lumbar spinous processes and the interspinous ligaments, and is continuous craniad with the aponeurosis of the serratus posterior inferior. Laterad it splits into three sheets, two for the above-named muscles and a third which passes to the vertebral transverse processes and forms the fascia covering the supraspinous muscles of the lumbar region. (2) By a similar aponeurosis from the ventral half of the iliac crest. (3) By fleshy fibres from the three crural arches. These are three delicate ligamentous arches which stretch from the crest of the ilium to the pubic spine. The dorsal one gives exit to the iliopsoas muscle, the middle to the femoral vessels, and the ventral to the spermatic cord. In the female the middle and ventral arches may fuse. The pillar between the dorsal and middle arches is attached to the iliopectineal eminence.

Insertion.—The fibres pass cranioventrad and end along a longitudinal line in a thin aponeurosis of insertion, which is united in the linea alba to those of the external oblique and transversus. At the caudal end of the aponeurosis all its fibres pass outside of the rectus abdominis (Fig. 73, k). At its cranial end the fibres divide into two sheets or laminæ, one of which passes outside of the rectus and the other inside. The outer lamina unites with the aponeurosis of the external oblique, while the inner lamina unites with that of the transversus. There is thus formed a sheath for the cranial part of the rectus muscle.

Relations.—Outer surface with the obliquus externus[155] (Fig. 68, p). Inner surface with the transversus abdominis (Fig. 69, l), and by its ventral tendon with the rectus.

Action.—Compressor of the abdomen.

M. transversus abdominis (Fig. 69, l).—A thin sheet covering the whole surface of the abdomen and lying beneath the internal oblique. Its fibres are nearly transverse.

Origin.—(1) By fleshy fibres or by a thin aponeurosis from the cartilages of all the false and floating ribs, by interdigitation with the fibres of the diaphragm. (2) From the tips of all the lumbar transverse processes. (3) From the ventral border of the ilium. (4) From the dorsal and middle of the three crural arches, where it may blend partly with the internal oblique. The muscle is continuous craniad with the transversus thoracis muscle. Near the lateral border of the rectus abdominis the muscle ends in a thin aponeurosis of insertion which is continued (its fibres having the direction of the muscle-fibres) to the

Insertion in the linea alba.

Relations.—Outer surface with the internal oblique (Fig. 73, o) and the rectus abdominis (Fig. 73, k), dorsad also with the longissimus dorsi (Fig. 69, f). Inner surface with a thin fascia covering the peritoneum and ventrocaudad with the rectus (Fig. 73, k). The dorsal edge of the muscle touches the iliopsoas and longissimus dorsi; the ventral edge touches the muscle of the opposite side.

Action.—Constrictor of the abdomen.

M. rectus abdominis (Fig. 73, k).—A rather thick, flat muscle which lies near the median ventral line separated by the linea alba from its fellow of the opposite side and stretching from the pubis to the first costal cartilage. Opposite the first lumbar vertebra it is approximately four centimeters wide. It narrows at both ends.

Origin by a strong tendon from the tubercle of the pubis. The muscle passes craniad at first between the peritoneal fascia and the transversus aponeurosis, then in the sheath formed by the internal and external oblique and the transversus aponeurosis. It emerges from the sheath opposite the xiphoid process and passes ventrad of the costal cartilages. Between the third[156] and fourth costal cartilages it ends in a thin tendon which passes beneath the transversus costarum and is

Inserted into the first costal cartilage near its middle, into the second costal cartilage near its sternal end, and into the sternum between the first and fourth cartilages.

Relations.—Outer surface of the cranial part of the muscle and its tendon with the pectoralis minor (Fig. 68, o) and the transversus costarum (Fig. 73, j); caudad the muscle is covered by the outer layer of the rectus sheath. Lateral edge with the obliquus externus (Fig. 68, p), obliquus internus (Fig. 73, o), and transversus abdominis (Fig. 69, l). Medial edge with the muscle of the opposite side. Inner surface with the internal intercostals (Fig. 69, k) and the rib cartilages; the inner layer of the rectus sheath and the peritoneum.

Action.—Retracts the ribs and sternum and compresses the abdomen.

IV. MUSCLES OF THE THORACIC LIMBS.

The muscles connecting the thoracic limbs with the rest of the body have been described.

1. Muscles of the Shoulder.

A. Muscles on the Lateral Surface of the Shoulder.

M. deltoideus.—The deltoid muscle of the cat is divided into two (or three) portions which are together equivalent to the human deltoid. These are the spinodeltoid, the acromiodeltoid, and possibly the clavobrachial, which is frequently called clavodeltoid.

M. spinodeltoideus (Fig. 75, e; Fig. 68, i).—A rather thick, flat muscle between the scapular spine and the deltoid ridge of the humerus. It forms a chord of the angle between the glenoid border of the scapula and the humerus.

Origin by short tendon-fibres from the glenoid border of somewhat more than the middle third of the spine of the scapula (Fig. 76, g), and from a tendinous raphe between the spinotrapezius (Fig. 68, j), acromiotrapezius (Fig. 68, h), and[157] infraspinatus (Fig. 75, c). The origin may pass toward the vertebral border of the scapula onto the infraspinatus muscle (Fig. 75, c).

Insertion (Fig. 81, h) by a flat tendon upon the deltoid ridge of the humerus, nearly parallel to that of the pectoralis major.

Relations.—Outer surface with the integument and at the insertion with the acromiodeltoideus (Fig. 75, f). Inner surface with the infraspinatus (c), teres minor (Fig. 80, c), caput laterale (Fig. 75, h), and caput longum (Fig. 75, g) of the triceps muscle.

Action.—Flexes the humerus and rotates it outward.

M. acromiodeltoideus (Fig. 75, f; Fig. 68, g).—A flat muscle which overlies the distal end of the spinodeltoideus (Fig. 75, c). It connects the acromion with the humerus.

Origin (Fig. 76, d).—From the glenoid border of the acromion, and sometimes the adjacent metacromion as far as the tip.

Insertion.—Mostly upon the outer surface of the spinodeltoideus (Fig. 75, c). The outer fibres are continued to the bone, especially at the lateral border of the muscle, and are inserted along a line ventrad of the line of insertion of the spinodeltoideus, and extending farther distally (Fig. 81, g). Some of the outer fibres pass into the brachialis (Fig. 80, h).

Relations.—Outer surface with the integument and the clavobrachial (Fig. 68, e). Inner surface with the infraspinatus (Fig. 75, c), teres minor (Fig. 80, c), spinodeltoid (Fig. 75, e), and caput laterale of the triceps (Fig. 75, h).

Action.—Like that of the spinodeltoid.

M. clavobrachialis (Fig. 65, k; Fig. 68, e).—A flat, triangular muscle on the cranial surface of the shoulder, forming a direct continuation of the clavotrapezius (Fig. 68, d). These two are frequently described as constituting a single muscle, the cephalohumeral or cephalobrachial. (The clavobrachial is frequently given the name clavodeltoid; as its homology with the human clavodeltoid appears doubtful, it seems well to use the name clavobrachial, as proposed by Clasen.)

Fig. 75.—Muscles on the Lateral Surface of the Arm.

a, M. supraspinatus; b, part of insertion of M. rhomboideus; c, M. infraspinatus; d, M. teres major; e, M. spinodeltoideus; f, M. acromiodeltoideus; g, caput longum of M. triceps brachii; h, caput laterale of M. triceps brachii; i, M. brachialis; j, M. biceps; k, M. brachioradialis; l, M. extensor carpi radialis longus; m, M. extensor communis digitorum (m′, tendon of same); n, M. extensor lateralis digitorum; (n′, tendon of same); o, M. extensor carpi ulnaris (o′, tendon of same); p, M. extensor indicis proprius; q, fifth head of M. flexor profundus digitorum; r, M. flexor carpi ulnaris. 1, vertebral border of the scapula; 2, spine of the scapula; 3, greater tuberosity of the humerus; 4, olecranon; 5, transverse ligament of the wrist.

 

Origin.—Its superficial fibres are continuations of the[158]
[159] clavotrapezius (Fig. 68, d); other fibres have origin from the clavicle and from a raphe laterad of the clavicle which is common to this muscle and the clavotrapezius. It passes distad along the cranial surface of the arm, growing narrower as it approaches the convexity of the elbow.

Insertion.—Eight to ten millimeters from the ulna it joins the brachialis muscle (Fig. 79, i) to be inserted with it by a flat tendon (Fig. 79, i; Fig. 87, c) upon a rough area on the medial surface of the ulna just distad of the semilunar notch, and about midway between the dorsal and ventral borders.

Relations.—Outer surface with the integument. Inner surface with the pectoralis major (Fig. 65, l), biceps (Fig. 77, g), lateral head of the triceps (Fig. 75, h), acromiodeltoid (Fig. 75, f), and brachialis (Fig. 80, h). Medial border with the pectoantibrachialis (Fig. 65, m). Lateral border free except near the insertion, where it is in relation with the brachialis.

Action.—Flexor of the antibrachium.

M. supraspinatus (Fig. 75, a) occupies the whole of the supraspinatus fossa of the scapula. It is covered by strong fascia which stretches from the free edge of the spine to the coracoid border of the scapula and to the coracoid half of its vertebral border.

Origin (Fig. 76, b) by fleshy fibres from the whole surface of the supraspinatus fossa, from the above-mentioned fascia, and from the subscapularis (Fig. 77, a) craniad of the coracoid border of the scapula.

Insertion (Fig. 83, a).—It passes over the capsule of the shoulder-joint, to which it is closely attached, and is inserted into the free border of the great tuberosity ventrad (or proximad) of the fossa for the infraspinatus (Fig. 83, c).

Relations.—Outer surface with the spinotrapezius (Fig. 68, j), acromiotrapezius (Fig. 68, h), levator scapulæ ventralis (Fig. 68, f), clavotrapezius (Fig. 68, d), and cleidomastoid (Fig. 65, h). Inner surface with the scapula. The distal end of the coracoid border is closely related to the pectoralis minor (Fig. 79, f). The glenoid border is related to the origin of the deltoidei.

[160]

Action.—Extends the humerus after it has been flexed on the scapula.

M. infraspinatus (Fig. 75, c).—This fills the infraspinatus fossa, its fibres converging to the insertion on the great tuberosity of the humerus.

Fig. 76.—Lateral Surface of the Scapula, with the Areas of Attachment of Muscles.

a, M. infraspinatus; b, M. supraspinatus; c, origin of M. biceps; d, M. acromiodeltoideus; e, M. teres minor; f, M. levator scapulæ ventralis; g, M. spinodeltoideus; h, M. acromiotrapezius; i, M. rhomboideus; j, M. teres major; k-l, line of insertion of M. spinotrapezius.

Origin (Fig. 76, a).—By fleshy fibres from the whole infraspinatus fossa, and by a raphe between it and the teres major (Fig. 75, d), and sometimes by fibres from the teres minor (Fig. 80, c), triceps, spinodeltoideus (Fig. 75, e), spinotrapezius (Fig. 68, j), and subscapularis (Fig. 77, a).

Insertion (Fig. 83, c).—By a flat tendon which passes over the capsule of the joint, into the ventral half of the infraspinatus fossa on the great tuberosity of the humerus. (The dorsal half of the fossa is covered by a synovial bursa.)

Relations.—Outer surface with the spinotrapezius (Fig. 68, j), spinodeltoid (Fig. 68, i), acromiodeltoid (Fig. 68, g), teres major (Fig. 75, d), and levator scapulæ ventralis[161] (Fig. 68, f). Inner surface with the scapula. Glenoid border with the teres minor (Fig. 80, c), the teres major (Fig. 75, d), and the long head of the triceps (Fig. 75, g).

Action.—Rotates the humerus outward.

M. teres minor (Fig. 80, c).—A small muscle from the glenoid border of the scapula to the proximal end of the humerus.

Origin (Fig. 76, e) by a sheet of tendinous fibres from the glenoid border of the scapula beginning about one-fifth the length of the border from the glenoid fossa and extending to its middle. It is often attached to the infraspinatus (Fig. 80, b) and the caput longum of the triceps (Fig. 75, g).

Insertion (Fig. 83, d) by a short tendon into the tubercle just distad of the infraspinatus fossa on the great tuberosity of the humerus.

Relations.—Outer surface with the spinodeltoideus (Fig. 75, e), acromiodeltoideus (Fig. 75, f), and the infraspinatus (Fig. 80, b). Inner surface with the lateral (Fig. 75, h) and long (Fig. 75, g) heads of the triceps and the capsule of the joint.

Action.—Assists the infraspinatus to rotate the humerus outward.

B. Muscles on the Medial Surface of the Shoulder.

(Fig. 77.)

M. subscapularis (Fig. 77, a).—A triangular mass from the subscapular fossa to the lesser tuberosity of the humerus.

Origin (Fig. 78, a).—From the whole subscapular fossa except along the fusiform area for the attachment of the levator scapulæ and the serratus anterior near the vertebral border, and except over a quadrangular area about one centimeter long near the glenoid angle. The origin is by fleshy fibres directly from the periosteum except along two or three lines marked by oblique ridges. To these lines are attached tendinous fibres. At the glenoid border the area of origin sometimes occupies part or the whole of the surface of the fossa for the origin of the teres major, so that the teres arises from the fascia on the surface of the subscapularis. At the coracoid border some of the fibres may take origin from the adjacent fascia of the supraspinatus (Fig. 77, d).

[162]

Insertion (Fig. 82, b).—The fibres converge to the glenoid border, and the insertion is by a strong, flat tendon into the dorsal border of the lesser tuberosity of the humerus.

Fig. 77.—Muscles on the Medial Side of the Arm.

M. epitrochlearis, M. clavobrachialis, and the breast-muscles have been removed. a, M. subscapularis; b, insertion of M. levator scapulæ and of M. serratus anterior; c, M. teres major; d, M. supraspinatus; e, M. latissimus dorsi (e′, cut edge of that part which becomes united with the bicipital arch); f, M. coracobrachialis; g, M. biceps brachii (the capsule of the joint has been laid open to show its tendon); h, cut insertions of pectoralis muscles; i, caput longum of M. triceps brachii; j, long portion of caput mediale of M. triceps brachii; k, intermediate portion of caput mediale of M. triceps brachii; l, short portion of caput mediale of M. triceps brachii; m, cut end of M. clavobrachialis; n, M. brachioradialis; o, M. extensor carpi radialis longus; p, M. extensor carpi radialis brevis; q, M. pronator teres; r, M. flexor carpi radialis; s, M. palmaris longus (s′, its tendons); t, M. flexor carpi ulnaris (t, ulnar head; t′, humeral head); u, third head of M. flexor profundus digitorum; u′, fifth head of M. flexor profundus digitorum; u″, one of the tendons of M. flexor profundus digitorum; v, M. pronator quadratus; w, M. abductor brevis pollicis; x, ulnar part of M. flexor sublimis digitorum. 1, transverse ligament of the wrist.

 

Relations.—Lateral surface with the scapula and the capsule of the shoulder-joint. Medial surface with the levator scapulæ (Fig. 73, h), serratus anterior (Fig. 73, i), part of the scalenus (Fig. 73, f), the transversus costarum (Fig. 73, j), and the coracobrachialis (Fig. 77, f). Cranial border with the supraspinatus (Fig. 77, d). Caudal border[163] with the teres major (Fig. 77, c) and infraspinatus (Fig. 75, c).

Action.—Pulls the humerus inward (mediad).

M. teres major (Fig. 75, d; Fig. 77, c).—A thick muscle, triangular in cross-section, lying parallel with the glenoid border of the scapula.

Origin (Fig. 76, j; Fig. 78, b) from the vertebral one-third of the glenoid border of the scapula, and from fascia covering the subscapularis (Fig. 77, a) and the infraspinatus (Fig. 75, c) over a small area near the glenovertebral angle of the scapula. It may touch the insertion of the rhomboideus.

Insertion (Fig. 82, f) by a tendon common to it and the latissimus dorsi (Fig. 79, d′), as already described.

Relations.—Outer surface with the caput longum (Fig. 75, g) and the long portion (Fig. 80, f) of the caput mediale of the triceps, the latissimus dorsi (Fig. 77, e), and the cutaneus maximus. Inner surface with the serratus anterior (Fig. 73, i), the scalenus (Fig. 73, f), the transversus costarum (Fig. 73, j), and the biceps (Fig. 77, g). Dorsal border with the subscapularis (Fig. 77, a) and infraspinatus (Fig. 75, c).

Action.—Rotates the humerus inward and flexes it in opposition to the infraspinatus, teres minor, and the deltoidei.

Fig. 78.—Medial Surface of the Scapula with the Areas of Attachment of Muscles.

a, M. subscapularis; b, M. teres major; c, M. serratus anterior; d, M. levator scapulæ; e, M. rhomboideus; f, M. occipitoscapularis; g, M. biceps; h, M. coracobrachialis; i, caput longum of M. triceps.

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2. Muscles of the Brachium or Upper Arm.

M. clavobrachialis and the pectoralis group (Fig. 65, page 109) lie partly in this region; they have been described.

M. coracobrachialis (Fig. 77, f; Fig. 79, c).—A very short muscle covering the inner (medial) surface of the capsule of the shoulder-joint. It extends from the coracoid process to the proximal end of the humerus.

Origin (Fig. 78, h).—By a round tendon from the tip of the coracoid process.