As specific differences we should add that the trapezius occupies a more or less extensive portion of the median and superior regions of the neck; terminating at a considerable distance from the head in the dog and horse, it, on the contrary, approaches it in the pig and in ruminants. The cervical portion, when it contracts, draws the scapula upwards and forwards, the dorsal portion draws it upwards and backwards. When the trapezius acts as a whole the scapula is raised.

The Latissimus Dorsi (Fig. 68, 4; Fig. 69, 5; Fig. 70, 5).—This muscle arises by an aponeurosis, the so-called dorso-lumbar aponeurosis, from the spinous processes of the last dorsal vertebræ (the seven last in the dog, fourteen or fifteen last in the horse), from the spinous processes of the lumbar vertebræ, and from the last ribs. Its fleshy fibres are directed downwards and forwards, being more oblique in direction posteriorly, and pass on the inner side of the posterior muscular mass of the arm, to be inserted into the internal lip of the bicipital groove of the humerus, or, a little lower down, on the median portion of the internal surface of the same bone. This latter mode of insertion is met with in the horse and the ox.

The anterior fibres cover the posterior angle of the scapula (as in man, where the corresponding angle, but in this case inferior, is covered by the same muscle), and, a little higher up, are in their turn concealed by a portion of the dorsal fibres of the trapezius. It covers, to a greater or less extent, the great serratus muscle. These relations are similar to those found in the human species.

We find that the fleshy fibres of the great dorsal are prolonged more or less backwards if we examine this muscle in the dog, the ox, the pig, and the horse. Indeed, the fibres reach to the thirteenth rib in the dog and the cat (that is to say, the last rib), the eleventh in the ox, tenth in the pig, and twelfth only in the horse. We say ‘only’ in connection with this last because it is necessary to remember that the ribs are eighteen in number on each side of the thorax of this animal, and that, accordingly, the fleshy fibres of the great dorsal muscle are, relatively, of small extent.

When this muscle contracts it flexes the humerus upon the scapula, and helps to draw the whole of the anterior limb backwards and upwards.

There is a muscular fasciculus which, because of its relations with the muscle we have just been studying, is known as the supplementary muscle of the latissimus dorsi. But as, on the other hand, this fasciculus is in relation with the triceps, we shall in preference consider it in relation with this latter (see p. 173).

The aponeurosis by which the great dorsal arises from the vertebral column covers, as in man, the muscles which occupy the grooves situated on each side of the spinous processes—the spinal muscles or common muscular mass, if we regard them as a whole (Fig. 70, 7); the sacro-lumbar and the long dorsal muscles covering the transverse spinal, if we consider them as distinct.

It would be superfluous to enter here into a detailed examination of these muscles.

If they are but little developed the spinous processes become prominent under the skin; if they are more so they may by their thickness project beyond the level of these processes, and these latter thus come to lie in a groove more or less marked, which, on account of the division which is determined by its presence, has caused the regions which it occupies to be designated by the names double back and double loins.

The muscles are extensors of the vertebral column.

Under the aponeurosis of the great dorsal muscle there is found in man another muscle, the serratus posticus inferior, which, on account of being deeply placed and its slight thickness, offers nothing of interest in connection with the study of external form. It arises from the spinous processes of the three last dorsal vertebræ and those of the three first lumbar; it then passes upwards and outwards, and divides into four digitations, to be inserted into the inferior borders of the four last ribs. We repeat that it is covered by the great dorsal muscle.

In the pig, ox, and horse, which have this latter muscle less developed in its posterior portion, the same small serratus muscle, known as the posterior serratus, is visible in the superficial layer of muscles (Fig. 69, 6; Fig. 70, 6). The number of its digitations is more or less considerable according to the species examined.

The Rhomboid Muscle (Fig. 70, 21).—In order to make intelligible the position of the rhomboid in the superficial layer in quadrupeds, it appears to us necessary to recall the anatomical characters of the muscle as found in man. The rhomboid arises from the inferior portion of the posterior cervical ligament, from the spinous process of the seventh cervical vertebræ and the four or five upper dorsal; thence passing obliquely downwards and outwards, it is inserted into the spinal border of the scapula, into the portion of this border which is situated below the spine; it sometimes extends to the middle of the interval which separates this latter from the superior internal angle of the same bone.

The portion of the muscle which arises from the cervical ligament and the seventh cervical vertebra is often separated from the lower portion by a cellular interspace. For this cause some anatomists have described the rhomboid as consisting of two parts—the superior or small rhomboid and the inferior or large rhomboid, on account of the position occupied by each, and of their difference in volume.

This muscle can only be seen in the region of the back, in the space limited externally by the spinal border of the scapula, below by the latissimus dorsi, and internally by the trapezius, which covers it in the rest of its extent. It is not in this space that it is seen in certain quadrupeds. As we pointed out in the section on osteology, the spinal border of the scapula is short, and it seems to be due to this limitation in length that the trapezius and the latissimus dorsi muscle are, at this level, in contact the one with the other in such a way that they fill up the interval in which the rhomboid is seen in man.

In the horse we can partly see it in the superficial muscular layer, but in the region of the neck only, at the superior border of the shoulder. Indeed, as we have already pointed out, the trapezius does not reach the occipital protuberance; for this reason a part of the anterior portion of the rhomboid may be seen—that is, the portion which corresponds to the superior part of the human muscle.

But whether it be covered by the trapezius, or, as we find in the cat and dog, by the mastoido-humeral muscle (see p. 150), which is very broad in this region, we do not the less recognise its presence; and in the horse and ox, in particular, it forms an elongated prominence beginning at the level of the scapula, and tapering as it ascends, towards the posterior part of the head.

Its origins are similar to those which we have already described in the human rhomboid. It arises from the cervical ligament and the spinous processes of the foremost dorsal vertebræ; its fibres converge and pass to the scapula, to be inserted into its superior or spinal border, or into the internal surface of the cartilage of prolongation.

It assists in keeping the scapula applied to the thoracic cage, and when it contracts, draws the scapula upwards and forwards.

Taking its fixed point at the scapula, it acts on the neck by its anterior fibres, and extends it.

We shall soon have occasion to mention this muscle again, in connection with the study of the muscles of the neck.

The Cutaneous Muscle of the Trunk (Fig. 71).—Immediately beneath the skin which covers the neck, shoulders, and trunk is found a vast cutaneous muscle, analogous to that which, in the human species, exists only in the cervical region.

This thin muscle, whose function is to move the skin which strongly adheres to it, and in this way to remove from it material causes of irritation (insects, for example), is of considerable thickness in the region of the trunk; where it constitutes what certain authors have designated by the name of panniculus carnosus. In this region it extends from the posterior border of the shoulder to the thigh, and, in the vertical direction, from the apices of the spinous process of the dorso-lumbar vertebræ to the median line of the abdomen.

Arising above from the supraspinous ligament of the dorso-lumbar and sacral regions (except in the carnivora; see below) by an aponeurosis which, posteriorly, covers the muscles of the hind-limbs, its fibres are directed to the elbow, on which they are arranged in two layers: a superficial, which becomes continuous with the panniculus muscle of the shoulder; and a deep, which passes on the inner side of the shoulder to be inserted into the internal surface of the humerus; this latter exists only in the dog and cat.

The most inferior fibres, behind, at the level of the knee-cap form a triangular process which in the horse receives the name of the stifle fold, from the name veterinarians give to the region of the articulation of the knee. This fold of skin, which commences on the antero-internal surface of this region, is directed upwards, and then forwards, to end by gradually disappearing over the corresponding part of the abdomen.

From this arrangement results a great mobility of the skin which covers the back. Further, it explains why it is possible to lift up this skin along with the panniculus which it covers, and to which it adheres, throughout the whole extent of the dorso-lumbar column. As we pointed out above, there is also a panniculus muscle of the shoulder and one of the neck. We will deal with them when treating of the regions to which those muscles belong.

The Coccygeal Region

As a sequel to the study of the muscles of the region of the trunk very naturally comes the description of those which, belonging to the region of the coccyx, are destined for the movements of the caudal appendix, of which this latter constitutes the skeleton. The muscles may not seem to be of much importance with regard to external form, but, as they form part of the superficial muscular layer, and as the mass of each is seen in the form of the tail in some animals (the lion, for example), they merit our attention for a moment. A few lines will suffice to give an idea of them. They are: the ischio-coccygeal, superior sacro-coccygeal, lateral sacro-coccygeal, and inferior sacro-coccygeal.

The Ischio-coccygeal (Fig. 18, 38; Fig. 69, 33; Fig. 70, 42).—This muscle, triangular in shape, better developed in the carnivora than in the horse, arises from the spine of the ischium, or from the supracotyloid crest, which replaces this latter in the solipeds and the ruminants. Thence its fleshy mass is directed upwards, expanding as it proceeds to be inserted into the transverse processes of the first two coccygeal vertebræ after insinuating itself between two of the following muscles, the lateral and inferior sacro-coccygeal.

In the dog and cat, the muscle is in great part covered by the great gluteal. In the ox, by a peculiar arrangement of the corresponding region of the muscles of the thigh—an arrangement which we will examine in connection with the study of the latter—it is more exposed than in the horse, and gives origin to an outline which corresponds to its general form in the region situated immediately below the root of the tail.

It is a depressor of the whole caudal appendix.

The Superior Sacro-coccygeal (Fig. 68, 39; Fig. 69, 34; Fig. 70, 43).—The fasciculi which form this muscle arise from the crest of the sacrum, and proceed thence to end successively on the coccygeal vertebræ. It is in contact in the middle line with the corresponding muscle of the opposite side.

It raises the tail and inclines it laterally; if the muscle of one side contracts at the same time as that of the other the tail is elevated directly.

The Lateral Sacro-coccygeal (Fig. 68, 40; Fig. 69, 35; Fig. 70, 44).—Situated on the lateral part of the caudal region, this muscle arises, in the dog, from the internal border of the iliac bone and the external border of the sacrum; in the horse, it arises from the crest of the sacrum. It is inserted into the coccygeal vertebræ.

It produces lateral movement of the tail.

The Inferior Sacro-coccygeal (Fig. 68, 41; Fig. 69, 36; Fig. 70, 43).—This muscle, which is fairly thick, arises from the inferior surface of the sacrum and the corresponding surface of the sacro-sciatic ligament; it is inserted into the coccygeal vertebræ.

It depresses the caudal appendix.

Muscles of the Neck

Mastoido-humeralis (Fig. 66, 3; Fig. 68, 9, 9, 10; Fig. 69, 12; Fig. 70, 15).—One of the most important muscles of the region of the neck in man is the sterno-cleido mastoid. We recollect that, in its inferior part, it is divided into two bundles, one of which arises from the manubrium of the sternum, and the other from the inner third of the clavicle, whence the denominations of the sternal portion and clavicular portion. The muscle formed by the union of these two portions is then directed obliquely outwards, backwards, and upwards, to be inserted into the mastoid process of the temporal bone and the two external thirds of the superior curved line of the occipital bone.

Now, the animals which we are here considering have but a rudimentary clavicle or are entirely without it. From the absence of this item of the skeleton there necessarily result modifications in the arrangement of the muscles of this region, which we must at the very outset explain, before undertaking the special study of the muscle which is the subject of the present paragraph.

Let us suppose, for the more definite arrangement of our ideas, that the clavicle is altogether absent, although we do find it in a rudimentary state in some animals and completely developed in others (marmot, bat), and we will proceed to indicate what this absence determines.

The great pectoral muscle in man arises in part from the clavicle; this origin not being possible in animals which have no clavicle, its attachments, as we have already seen, are concentrated on the sternum. The trapezius in man similarly arises in part from the clavicle; for the reasons above indicated its clavicular fasciculi cannot exist in distinct form in the animals which have no clavicle.

The sterno-cleido mastoid, whose inferior attachments we mentioned above, cannot have a clavicular portion.

It is the same in the case of the deltoid, which, we know, arises in part from the anterior bone of the shoulder.

Of the four muscles which have partial clavicular origins in man, two are known to us in connection with animals—the great pectoral and the trapezius. What has become of the other two, the sterno-cleido mastoid and the deltoid?

It is this which we now proceed to investigate. After a fashion simple enough, but which it is necessary to describe, the clavicular fasciculi of the trapezius and the corresponding fasciculi of the sterno-cleido mastoid are united the one to the other; the portion of the deltoid which in man arises from the clavicle, by reason of the absence of this latter, is also combined with the fleshy mass formed by the preceding muscles. From this fusion results the muscle known as the mastoido-humeral. This muscle, which consists of a long fleshy band situated on the lateral aspect of the neck, takes its origin, as a general rule, from the posterior surface of the skull and the upper part of the neck, from which it passes obliquely downwards and backwards, covering the scapulo-humeral angle—that is, the region known as the point of the shoulder or arm—and is inserted into the anterior border of the humerus, the border which, limiting anteriorly the musculo-spiral groove, forms a continuation of the deltoid impression. On account of the regions with which it is related, Bourgelat named this muscle the muscle common to the head, neck, and arm.

It is at the level of the scapulo-humeral angle that the vestiges of the clavicle are found.

This bone is represented in some animals—the pig, ox, and horse—by a single tendinous intersection, more or less apparent, which extends transversely from the scapula to the anterior extremity of the sternum. In the dog and the cat, we find, besides, on the deep surface of the muscle and at the level of this tendinous intersection, the rudiment of the clavicle of which we made mention in the section on Osteology (see p. 25).

It is beneath the intersection, the existence of which we have just pointed out, that is found that portion of the mastoido-humeral muscle which corresponds to the clavicular fasciculi of the deltoid; that portion which is situated above the intersection corresponds to the clavicular fibres of the sterno-cleido-mastoid and of the trapezius.

The mastoido-humeral presents certain varieties in different animals.

In the dog and the cat, this muscle, which is blended above with the sterno-mastoid (see p. 153), to be inserted with it into the mastoid process and the mastoid crest, covers the neck for a considerable extent from the superior curved line of the occipital bone to which it is attached, to the trapezius with which it unites posteriorly, but from which it separates below. Between these two extreme points of its superior portion it is attached to the cervical ligament.

In the pig and in ruminants, in which the trapezius approaches more closely to the head, the mastoido-humeral occupies, in consequence, a less extent of the cervical region.

In the horse, the mastoido-humeral neither covers the neck nor joins the trapezius; indeed, we have already shown that it is separated by a considerable distance from the head. In the limited interval between these two muscles a part of the rhomboid and parts of other muscles are seen with which we shall soon be occupied.

This muscle, as regards the horse, is described by some anatomists as consisting of two parts: one anterior, or superficial; the other posterior, or deep. In reality, the first only corresponds to the mastoido-humeral, which we are considering; the posterior may be more exactly regarded as representing a special muscle of quadrupeds, but which is here a little deformed, the omo-trachelian (see p. 155).

When the mastoido-humeral contracts, taking its fixed point above, it acts as an extensor of the humerus, and carries the entire fore-limb forwards. If it takes its fixed point below—that is to say, at the humerus—it inclines the head and neck to its own side. If it contracts at the same time as the mastoido-humeral of the opposite side, then the head and the neck are carried into the position of extension.

The Sterno-mastoid (Fig. 66, 5; Fig. 68, 11; Fig. 69, 15; Fig. 70, 17).—Having described the clavicular portion of the sterno-cleido-mastoid in connection with the mastoido-humeral, because it forms a part of the latter, we have, in order to complete the homologies of this muscle, to study now that which corresponds to its sternal portion. This is the sterno-mastoid muscle. In all the quadrupeds with which we are here concerned this muscle arises from the anterior extremity of the sternum; narrow and elongated in form, it passes towards the head in a direction parallel to the anterior border of the mastoido-humeral, from which it is separated by an interspace which, along its whole length, lodges superficially the jugular vein; hence the name of jugular groove, which is given to this part of the neck (Fig. 10, 18).

It is inserted, in the case of the dog and cat, into the mastoid process, where it is united with the mastoido-humeral; in the ox it is divided into two portions—one which goes to the base of the occipital bone, the other passing in front of the masseter is by the medium of the aponeurosis of this latter attached to the zygomatic crest. This latter part is considered by some writers as forming a portion of the panniculus muscle of the neck.

In the horse it is attached to the angle of the lower jaw by a tendon, which an aponeurosis that passes under the parotid gland binds to the mastoido-humeral muscle and the mastoid process.

By reason of this insertion into the jaw, in the case of the solipeds, this muscle is further named the sterno-maxillary.

When it contracts, it flexes the head, and inclines it laterally. This movement is changed to direct flexion when the two sterno-mastoid muscles contract simultaneously.

In man, the sterno-cleido-mastoid and the trapezius leave a triangular space between them, which, being limited inferiorly by the middle third of the clavicle, is known as the supraclavicular region; this region, being depressed, especially in its inferior part, has also been given the name of supraclavicular fossa—popularly called the ‘salt-cellar.’

The muscles which form the floor of this region, passing from above downwards, are: a very small portion of the complexus, splenius, levator anguli scapulæ, posterior scalenus, and anterior scalenus; then, crossing these latter, and most superficial, is the omo-hyoid muscle.

An analogous region, but of only slight depth, exists in quadrupeds; its borders are formed by the mastoido-humeral and trapezius muscles.

It is not limited below by the clavicle—we know, indeed, that this, or the intersection which represents it, belongs to the mastoido-humeral muscle—but by the inferior portion of the spine of the scapula.

It is of greater or less extent according to the species considered.

In the dog, cat, pig, and ox, it is narrow, for the muscles which bound it approach one another pretty closely. It has, as in man, the form of a triangle, with the apex above. In the horse it is much broader, and, contrary to the arrangement which it presents in the human species, the widest part is directed upwards.

The muscles which we find there are, consequently, more or less numerous. In the dog and cat they are: a portion of a muscle which we do not normally meet with in man—the omo-trachelian—then in a decreasing extent: supraspinatus, levator anguli scapulæ and splenius.

In the pig: the omo-trachelian, supraspinatus, and the terminal portion of the sterno-prescapular—the anterior part of the lesser or deep pectoral muscle.

In the ox: the omo-trachelian only.

But in the horse we find the omo-trachelian, the supraspinatus, and the terminal extremity of the sterno-prescapular; then in a larger extent of area the levator anguli scapulæ and the splenius; and, finally, the anterior portion of the rhomboid.

Among the muscles which we have just enumerated are some that we have already studied; these are the sterno-prescapular and the rhomboid. We will examine the supraspinatus muscle in connection with the region of the shoulder.

As to the scaleni muscles and the complexus, they are deeply situated, whereas the omo-hyoid is visible in the anterior region of the neck only.

There remain for us, accordingly, to examine, at the present juncture, but the omo-trachelian, levator anguli scapulæ, and splenius muscles.

The Omo-trachelian Muscle (Fig. 68, 13; Fig. 69, 17; Fig. 70, 20).—Also called the acromio-trachelian, levator ventri scapulæ,[21] the angulo-ventral muscle, and the transverso-scapular,[22] etc., this muscle is described by some hippotomists as belonging to the mastoido-humeral, of which it then forms its posterior or deep portion (see p. 153).

The omo-trachelian muscle is found in all mammalia, man alone excepted. It is, however, sometimes found in the human being; but it then constitutes an anomaly.

In the dog, pig, and ox, it arises from the inferior part of the spine of the scapula, in the region of the acromion, and terminates on the lateral portion of the atlas.

In the cat it is attached besides to the base of the occipital bone. It is visible in the space limited by the trapezius and the mastoido-humeral, the direction of which it crosses obliquely.

In the horse it appears to be blended in clearly defined fashion with the mastoido-humeral. Attached below, like this latter, to the anterior border of the humerus, it covers the scapulo-humeral angle; and is attached by its upper portion to the transverse processes of the first four cervical vertebræ.

We remember that the transverse processes are often, from their relation with the trachea, known as the tracheal processes. Hence the word ‘trachelian,’ which forms part of the name of the muscle with which we are now dealing.

By its contraction it helps to draw the anterior limb forwards.

When this muscle, as an abnormality, exists in man, it arises from the clavicle or the acromion process, traverses the supraclavicular fossa, and is inserted into the transverse processes of the atlas or axis, or of both these vertebræ, or of the cervical vertebræ below these latter. It is then known by the names of the elevator of the clavicle or elevator of the scapula, and, finally, as the cleido-omo-transversalis (Testut).[23]

The Levator Anguli Scapulæ (Fig. 68, 15; Fig. 70, 23).—As we have pointed out (p. 136), the levator anguli scapulæ, because of its connections with the great serratus, is sometimes described with it. But inasmuch as in human anatomy these two muscles are considered separately, and that, in the superficial layer of muscles, they are seen in different regions—the great serratus in the thoracic, and the levator anguli scapulæ in the cervical—we prefer to study them separately.

We remember that in man this muscle arises from the transverse processes of the upper cervical vertebræ and is inserted into the superior portion of the spinal border of the scapula, into the portion of this border which is situated above the spine; it also contributes to the formation of the floor of the supraclavicular region.

When it contracts, it draws the superior portion of the scapula forwards and upwards, and causes a see-saw movement, for at the same time the inferior angle of the scapula is directed backwards. Taking its fixed point at the shoulder, it directly extends the neck if the muscle of one side acts at the same time as that of the opposite; but if only one muscle contracts it inclines the neck to the corresponding side.

It is to be noticed that during movements a little more active than the ordinary the levator anguli scapulæ, as moreover the other muscles of the neck do, becomes very distinct. We have, indeed, often remarked that, apart from these movements, each time the support of one of the fore-limbs is brought into requisition a brusque contraction of the muscles of this region accompanies it.

This contraction gives the impression that, as on the one hand, each support determines a momentary arrest of progression, a jolt, and on the other hand, the head continues to be projected in the forward direction, the latter should be retained. But it cannot be so except by an effort in the opposite direction—that is to say, by the brusque contraction which we have just pointed out.

Analogous contractions also take place in a man while running at the beginning of each contact of the lower limbs with the ground.

We may add, apropos of this latter, that displacements of the head, sometimes in very pronounced fashion, take place during simple walking, and that every time one of the lower limbs is carried forwards the head is projected in the same direction. These displacements, which we also find take place in the horse in pacing, especially in the region of the neck and head, seem then to have the effect of aiding the progression of the body forwards.

They occur especially in animals when drawing a heavy load, and in individuals whose walking movements are executed with difficulty.

It is necessary to repeat that, in these cases, the individual appears to assist the movement of his body by the impetus which the projection of his head forward determines, in order to add—and it is for this that we have referred to the subject—that during the intervals between each projection the head is carried backwards by a muscular contraction similar to that above discussed.

The Splenius (Fig. 68, 14; Fig. 70, 22).—In man, this muscle is attached in the median line to the inferior half or two-thirds of the posterior cervical ligament, to the spinous processes of the seventh cervical, and four or five upper dorsal vertebræ; it passes obliquely upwards and outwards, becomes visible in the supraclavicular region, passes under the sterno-cleido-mastoid, and proceeds to duplicate the cranial insertions of this latter; and, further, the most external fasciculi of this muscle are inserted into the transverse processes of the atlas and the axis.

These separate superior attachments, and the division of the muscle which results, have caused the splenius to be regarded as formed of two portions: splenius of the head, and splenius of the neck.

In the horse, this muscle, which is of voluminous dimensions, arises from the superior cervical ligament, and the spinous processes of the first four or five dorsal vertebræ; thence it proceeds to be inserted into the mastoid crest, and the transverse processes of the atlas and three or four vertebræ following.

The region occupied superficially by the splenius is remarkable for the prominence which this muscle, with the deeply-seated complexus, which is equally bulky, determines at this level; it is situated above that region of the neck, in which are seen in part the fasciculi of the levator anguli scapulæ. It terminates above and in front in the ridge, which is sometimes very pronounced, which the transverse processes of the atlas make on each side of this part of the neck.

In the dog and the cat, the superior and anterior region of the neck is thick and of rounded form, on account of the development which the splenius presents in those animals; but it is covered by the mastoido-humeral.

This latter relation is also found in the ox, but the splenius in this case is but slightly developed.

When the splenius contracts it extends the head and neck, while inclining them to its own side.

If the splenius of one side contracts at the same time as that of the opposite, the extension takes place in a direct manner—that is to say, without any modifying lateral movement.

Infrahyoid Muscles

Having studied the lateral surfaces of the neck, we must now examine the anterior part of this region. Here, between the two sterno-mastoid muscles, we find a space broader above than below, in which are situated the larynx and the trachea, to the general arrangement of which is due the cylindrical form which this region presents. This space corresponds to that which in the neck of man is limited laterally by the sterno-cleido-mastoid muscles, below by the fourchette of the sternum, and above by the hyoid bone. In animals, as in man, it is called the infrahyoid region.

The hyoid bone in quadrupeds is situated between the two rami or branches of the lower jaw. Owing to this disposition, the region above this bone, instead of having its surface projecting a little beyond the inferior border of the maxillary bone, is depressed. This is especially so in the horse. It is there that we find in this animal the region known as the trough (auge); the larynx corresponds to that part known as the gullet.

The muscles which occupy the infrahyoid region are: the sterno-thyroid, the sterno-hyoid, and the omo-hyoid. There is also a thyro-hyoid, but because of its deep situation and its slight importance it offers no interest from our point of view.

Sterno-thyroid and the Sterno-hyoid Muscles.—These two muscles, long, narrow, and flat, arise from the anterior extremity of the sternum; then, covering the anterior surface of the trachea, they proceed to terminate, the one on the thyroid cartilage, and the other on the hyoid bone. The sterno-hyoid is superficial; it covers the sterno-thyroid, which, however, projects a little on its outer side.

Omo-hyoid.—This muscle does not exist in the dog or cat. It arises, in the horse, from the cervical border of the scapula, where it blends with the aponeurosis that envelops the subscapularis muscle, but in the pig and the ox it arises from the deep surface of the mastoido-humeral muscle. It is directed obliquely upwards and inwards, becoming superficial at the internal border of the sterno-mastoid, and is inserted into the hyoid bone.

The region in which are united the portion of the neck which we have just studied and the neighbouring part of the thorax—that is, the breast—has certainly, in our opinion, a form less expressive than the corresponding region in man.

In the latter, indeed, the fourchette of the sternum, with the hollow which it determines, the heads of the clavicles, and the sterno-cleido-mastoid muscles, by the elevations which they produce, and the trachea, by the situation which it occupies in the inferior part, constitute a whole in which are admirably indicated, not only the forms of the organs which constitute this region, but also the relations which these organs have one with another; and, to a certain extent, their respective functions.

In making an exception in the case of the ox, in which a fold of skin, the dewlap, which passes from the neck to the breast, constitutes an element of form which possesses some expressive value; in the horse and in the dog, which possess no sternal fourchette and no heads of clavicles, the bones and the muscles are found nearly on the same plane. This produces a uniformity which is evidently inferior, from an æsthetic point of view, to the modelling of the corresponding region of the human body. Such, at least, is our impression.

Suprahyoid Muscles

As their name indicates, these muscles are found above the hyoid bone; amongst those which should arrest our attention for a moment are the mylo-hyoid and the digastric.

Mylo-hyoid.—This muscle, forming a sort of fleshy sling which contributes in great measure to form the floor of the mouth, is situated between the lateral halves of the inferior maxillary bone. Arising on each side from the internal oblique line of the mandible, its fibres are directed towards the median line, to be inserted posteriorly into the hyoid bone, and, between this bone and the anterior part of the mandible, into a median raphe which unites these latter.

Digastric.—This muscle arises from the styloid process of the occipital bone and from the jugular process; it thence passes downwards and forwards, and terminates variously, in different species. In the ox and the horse it terminates in its anterior portion on the internal surface of the inferior maxillary bone, close to the chin. But in the horse a bundle of fibres is detached from the upper portion of the muscle, to be inserted into the recurved portion of the jaw. It is to this fasciculus that Bourgelat has given the name of ‘stylo-maxillary muscle.’

In the pig, dog, and cat, the digastric differs more from the corresponding muscle in man; it is not, as in the latter, formed of two parts. The anterior portion only exists. This consists of a thick muscular mass, which is inserted into the middle of the internal surface of the lower jaw.

In the dog and cat it is clearly recognisable in the superficial layer of muscles by the long and thick prominence which it produces below the masseter, against the inferior border of the mandible (see pp. 235 and 237, the two figures showing the myology of the head of the dog).

By its contraction, it draws the lower jaw downwards and backwards.

Panniculus of the Neck.—This very thin muscle, which cannot be recognised on the exterior, calls for little notice.

We shall merely point out that it duplicates the skin of the cervical region; but as the latter is only slightly adherent to it, the panniculus of this region seems rather destined to maintain in position the muscles which it covers than to displace the cutaneous covering.

We recall the fact that in man, on the contrary, the muscle is very evident at the instant of its contraction, and, for this reason, it presents a very great interest with regard to external modelling, and it plays an important part in the expression of the physiognomy.

MUSCLES OF THE ANTERIOR LIMBS

Muscles of the Shoulder

Deltoid (Fig. 68, 16, 17; Fig. 69, 18; Fig. 70, 24).—This is the first muscle we study in connection with the shoulder in human anatomy. Indeed, its wholly superficial position, and especially the manner in which it is separated from the surrounding muscles, its volume, and its characteristic modelling, give it such an importance that, from the didactic point of view, there is every indication for commencing with this muscle in studying the region to which it belongs. If, in regard to quadrupeds, we also commence with it, it is merely in deference to the spirit of method, and for the sake of symmetry; for it is far from presenting, in the latter, characters so distinctive and so clearly defined.

It is necessary to remark, at the outset, that in quadrupeds, on account of the absence or slight development of the clavicle, the clavicular portion of this muscle is, as we have shown, united to bundles of the same kind belonging to the sterno-cleido-mastoid and trapezius to form the mastoido-humeral (see p. 151). There exists, therefore, in an independent form, the scapular portion only.

It is this latter which, by itself alone, forms the deltoid of quadrupeds, a muscle known, in veterinary anatomy, as the long abductor of the arm.

In the dog and the cat it consists of two parts, one of which arises from the spine of the scapula; the other from the acromion process. Thence it passes to the crest of the humerus, which limits the musculo-spiral groove anteriorly, to be attached at a point which is found, as in other quadrupeds, to be the homologue of the human deltoid impression, or deltoid V, of the human humerus.

In the ox, in which the acromion process, which is very rudimentary, does not attain the level of the glenoid cavity, the acromion portion is but slightly marked off from that which takes its origin from the spine of the scapula.

Still, in the horse, which is completely deprived of an acromion process, the deltoid muscle is correspondingly divided into two parts, separated from one another by superficial interstices, but of which the arrangement differs from that of the portions above indicated; one part, the posterior, arises above from the superior part of the posterior border, and the postero-superior angle of the scapula (exactly as if, in man, certain fasciculi of the deltoid took their origin from the axillary border and inferior angle of the scapula); the other, anterior, arises from the tuberosity of the spine of the same bone. The two parts, united inferiorly, proceed to be inserted into the deltoid impression or infratrochiterian crest of the humerus.

It is necessary to add that the deltoid is inserted into the humerus, above the insertion of the mastoido-humeral.

This muscle flexes and abducts the humerus, and also rotates it outwards.

With regard to the other muscles of the human shoulder, subscapularis, supraspinatus, infraspinatus, teres minor and teres major, they are also present in quadrupeds, but in a form more elongated, as the scapula has its dimensions more extended from below upwards—that is, from the glenoid cavity towards the superior or spinal border.

Subscapularis.—This muscle occupies the subscapular fossa, from which it takes its origin, leaving free the superior part where the surface is found, to which are attached the serratus magnus and the levator anguli scapulæ. It passes towards the arm, to be inserted into the small tuberosity of the humerus. It is an adductor of the arm.

The subscapularis does not offer any interest from the point of view of external form, for it is completely covered by the scapula.

We speak of it, however, because we mention it in human anatomy, and that it affords us here a new opportunity of bringing into prominence the differences which exist in connection with the mobility of the shoulder.

We remember that in man, when the arm is abducted, and then raised a little above the horizontal, the scapula see-saws, is separated, to a certain extent, from the thoracic cage inferiorly and externally, and that, on the superficial layer of muscles, we are then able to see in the bottom of the armpit, at the level of the deep portion of the posterior wall of the latter, a small part of the subscapularis muscle.

In the animals with which we are here occupied it is not the same; for they are incapable of performing with their fore-limbs a movement analogous to that to which we have just referred, the humerus in their case being retained in contact with the trunk by the muscular masses which surround it.

Supraspinatus (Fig. 68, 19; Fig. 70, 25; Fig. 72, 7).—This muscle, as its name indicates, occupies the supraspinous fossa—that is to say, that which, by reason of the direction of the scapula in quadrupeds, is situated in front of rather than above the spine. It arises from this fossa; and, further, from the external surface of the cartilage which prolongs the scapula upwards in solipeds and ruminants. It projects more or less beyond the supraspinous fossa in front.

After passing downwards towards the humerus, it is inserted into the summit of the great tuberosity or trochiter—that is to say, to a part of this osseous prominence which represents the anterior facet of the great tuberosity of the human humerus, into which, as we know, the corresponding muscle is inserted.

In solipeds and ruminants it is inserted, by a second fasciculus, into the small tuberosity.

In the pig and the horse its anterior border is in relation with the terminal portion of the sterno-prescapular anterior portion of the small or deep pectoral.

The supraspinatus, which in man is completely covered by the trapezius, is partly visible in the superficial layer of the cat, dog, pig, and horse, in the lower part of the space limited by the mastoido-humeral and the trapezius. It is crossed by the scapulo-trachelian.

It is, in the ox, completely covered by these muscles, but its form, notwithstanding this, is easily discerned by the prominence which it produces. When it contracts, the supraspinatus muscle carries the humerus into the position of extension.

Infraspinatus (Fig. 68, 20; Fig. 72, 8).—This muscle, which occupies the infraspinous fossa, which, in quadrupeds, is situated behind the spine of the scapula, arises from the whole extent of this fossa, and in solipeds and ruminants encroaches on the cartilage of prolongation. Its fibres are directed downwards and forwards, to be inserted into the great tuberosity of the humerus—the trochiter—below the insertion of the supraspinatus.

It is completely covered (ox and horse), or in part only (cat and dog), by the portion of the deltoid which arises from the spine of the scapula; nevertheless, its presence is revealed by the prominence which it produces.

It is an abductor and external rotator of the humerus.

In connection with this muscle, which, as we have just pointed out, is less seen in the superficial muscular layer than the supraspinatus, we will draw attention to the fact that this arrangement is exactly the reverse of that which is found in the human shoulder. In this latter it is the supraspinatus which is not visible; while, on the contrary, the infraspinatus is uncovered in a considerable part of its extent. We further notice that it is accompanied by the teres minor, and that the teres major, situated inferiorly, forms with these two muscles a fleshy mass which, below, ends on the superior border of the great dorsal muscle.

In quadrupeds, in which the infraspinatus is so slightly visible, the teres major and minor are not found at all in the superficial muscular layer.

Accordingly, we will say but few words about them.