Artistic Anatomy of Animals Part 19

=Teres Major.=--This muscle is known to veterinarians as _the abductor of the arm_; it arises from the postero-superior angle of the scapula (the inferior angle of the human scapula), from which it pa.s.ses to be inserted into the internal surface of the humerus.

It is covered by the latissimus dorsi and the posterior muscular ma.s.s of the arm.

In brief, for the better understanding of the relations of the teres major and minor muscles in quadrupeds, we may fancy the corresponding muscles in man modified in the following manner: The infraspinatus, thicker, covering the teres minor; latissimus dorsi, more extended in its superior part, covering a large proportion of the teres major. As to the relations of the teres minor with the deltoid, they exist in man, seeing, in this case, the same muscle is, in its external portion, covered by this latter. With regard to the relations of the teres major with the posterior muscular ma.s.s of the arm, they also exist in man, since the external surface of this muscle is covered by the triceps.

These modifications are sufficient to render the small and large teres muscles completely invisible in the superficial layer.

The muscles of the shoulder which we have just been studying fulfil, with regard to the articulation which they surround, the function of active ligaments. This role is made necessary by the laxity of the scapulo-humeral capsule--a laxity which renders it incapable by itself of maintaining the bones in contact at this joint.

The same condition exists in man.

=Panniculus Muscle of the Shoulder.=--This thin muscle covers, as its name implies, the region of the shoulder, and is the continuation forward of the panniculus muscle of the trunk.

It arises, by its superior part, from the region of the withers and from the superior cervical ligament; thence its fibres descend directly towards the elbow, to terminate at the level of the region of the forearm.

The muscle is not found in the pig or in the carnivora.

Muscles of the Arm

We should remember, at the outset, that in man the muscles of the arm are divided into two groups: one anterior, which contains the biceps, brachialis anticus, and the coraco-brachialis; the other, posterior, which is const.i.tuted by a single muscle, the triceps.

In animals, we find them in the same number and arranged in a.n.a.logous fas.h.i.+on--that is to say, in two groups--with respect to the bone of the arm. But then we find that they have undergone a transformation with regard to their length, and it is the change of general aspect which results from this modification that we proceed to examine.

We know that in quadrupeds, and especially in the domestic animals, the humerus is relatively short in proportion to the forearm. We have already seen, in dealing with the bones, that whilst in the human species the humerus is longer than the forearm, in the dog and cat these two segments of the fore-limb are of equal length, and that the humerus of the horse is, on the contrary, much shorter. Now, let us suppose the human humerus to be shorter than it is in reality; the anterior muscles undergoing, very naturally, the same reduction, will be uncovered only slightly by those above--the deltoid and the great pectoral--or will remain completely hidden by them. Thus would be found realized the disposition which we meet with in quadrupeds of the muscles of this region.

With regard to the posterior muscular ma.s.s of the arm, it does not undergo the same change. The muscle which const.i.tutes it--the triceps cubiti--occupies, on the contrary, a greater area. Let us suppose, further--for it is the best method of comprehending the h.o.m.ologies which now occupy our attention--the humerus of man to be shortened as before, and directed downwards and backwards (as in quadrupeds), this bone would form an acute angle with the axillary border of the scapula. Let us suppose also that the long portion of the triceps, instead of arising solely from the superior part of this axillary border, is attached to the whole length of the latter, and that the triceps fills the whole interior of the angle formed by the arm and the shoulder. We then shall have an idea of what the triceps is in quadrupeds. It is necessary to add that the general resemblance would be still more complete if the arm were firmly supported by the side of the thorax, because in quadrupeds it occupies an a.n.a.logous position, determined by the arrangement of the muscles which, proceeding from the trunk and neck, are attached to it.

Anterior Region

=Biceps Cubiti= (Fig. 68, 21; Fig. 72, 10, 11).--This muscle, also called _the long flexor of the forearm_, does not merit the name except by its a.n.a.logy with the corresponding muscle in man. Indeed, in the domestic animals it is not divided into two parts; it is represented by a single fasciculus, long and fusiform, situated on the front of the humerus, and directed obliquely downwards and backwards, as the latter, on its part, is also inclined.

It arises above from a tubercle at the base of the coracoid process, which surmounts the glenoid cavity of the scapula. Its tendon, which is highly developed in the solipeds, occupies the bicipital groove. We remember that in these latter the groove in question is divided into two channels by a median prominence.

The tendon in which the muscle ends is inserted into a tuberosity, situated on the internal surface of the superior extremity of the radius--the bicipital tuberosity. In the pig, the cat, and the dog, there is detached from the tendon to which we have just referred a fasciculus of the same nature, which, after having wound round the radius, is inserted into the internal surface of the ulna, towards the base of the olecranon process. From the inferior part of the muscle arises a fibrous band, comparable to the aponeurotic expansion of the human biceps; but, instead of pa.s.sing downwards and inwards, as does the latter, it terminates on the muscular ma.s.s which const.i.tutes the antero-external part of the forearm.

The biceps is not seen in the superficial layer, except in the dog and cat (in which the humerus is, in fact, proportionately long); and even in them only to the slightest extent. It is covered partly in these latter, and completely in other animals, by the great pectoral and the inferior portion of the mastoido-humeral--that is to say, that part of the latter which represents the whole of the clavicular fibres of the human deltoid.

The biceps is a flexor of the forearm on the arm. It also contributes to the movement of extension of the humerus.

=Brachialis Anticus= (Fig. 68, 22; Fig. 69, 19; Fig. 70, 27; Fig. 72, 12).--In veterinary anatomy further designated as _the short flexor of the forearm_, this muscle, which is thick, occupies the musculo-spiral groove, and arises from it, reaching upwards to just below the head of the humerus. But it does not, as in man, extend to the internal surface of the bone.

Situated on the outside of the biceps, it is directed towards the forearm, and terminates by a flattened tendon, which, dividing into two slips, pa.s.ses below the bicipital tuberosity, on the internal surface of the radius, into which one of these slips is inserted, while the other proceeds to terminate on the ulna.

The inferior half of this muscle is visible on the superficial layer, in the s.p.a.ce limited posteriorly by the triceps brachialis, and below by the muscles of the forearm, which correspond to the external muscles of the human forearm, and in front by the great pectoral and the mastoido-humeral. It is in the upper part of the inters.p.a.ce which separates these latter from the brachialis anticus that the deltoid insinuates itself to proceed to its insertion into the humerus.

These relations precisely recall those which we meet with when we examine the external surface of the human arm, with this difference, however--that in the latter the anterior brachialis anticus is extensively related, in front, to the biceps. However, in animals it is not absolutely the same, since, as we have shown above, the biceps is covered, more or less completely, by the mastoido-humeral and the great pectoral.

The brachialis anticus flexes the forearm on the arm.

=Coraco-brachialis.=--In man this muscle, which occupies the superior half, or third, of the internal surface of the humerus, is visible only when the arm is abducted, and then especially when it approaches the vertical position; indeed, it is only in this att.i.tude that the region which it occupies is accessible to view.

But an a.n.a.logous att.i.tude not being possible in domestic animals, in which the arm is fixed along the corresponding parts of the trunk, the result is that the coraco-brachialis is always covered, and that, consequently, it presents nothing of interest from our point of view. We speak of it, then, merely in order to complete the series of the muscles of the anterior surface of the arm, among which we rank it, in spite of the fact that in veterinary anatomy it is described as a muscle of the shoulder.

It arises above from the coracoid process, and thence pa.s.ses downwards towards the internal surface of the humerus into which it is inserted, more or less high up, according to the species. The coraco-brachialis is an adductor of the arm.

Posterior Region

=Triceps Cubiti= (Fig. 68, 23, 24; Fig. 69, 20, 21; Fig. 70, 28, 29; Fig. 72, 13, 14, 15, 16).--This muscle, which is voluminous in the quadrupeds with which we are here concerned, fits more or less completely the angular s.p.a.ce between the scapula and the humerus. Its bulk forms a thick prominence, which surmounts the elbow and the forearm.

We should say, with regard to this ma.s.s, that if the deltoid does not const.i.tute in quadrupeds a prominence sufficient to remind one of that which this muscle produces in man, the triceps, in producing an a.n.a.logous elevation, seems to replace in the general form of the body the relief which the deltoid is incapable of producing.

The triceps is divided into three portions, which, as in man, have the names middle, or long head; external and internal heads. But that which renders the nomenclature a little complicated is that veterinary anatomists have given other names to these three parts: that of _great extensor of the forearm_ (caput magnum) to the long head; _the short extensor of the forearm_ (caput parvum) to the external head; and of _medium extensor of the forearm_ (caput medium) to the internal.[24]

[24] Other names given by certain authors to the parts of this muscle which we have just enumerated still further complicate this nomenclature.

The long head is further designated by them under the names of the _long_ or _great anconeus_; the _external head_ under those of _external anconeus_, or _lateral_ or _short anconeus_; whilst the internal head becomes the _internal anconeus_, or _median_.

It is more especially the long portion and the external head which, being visible on the external surface of the arm, contribute to the external form.

The long portion, which is triangular in shape and of considerable development, arises in the cat and the dog from the inferior half or two-thirds of the posterior border of the scapula (axillary border); from the whole extent of that border as far as the superior posterior angle in the pig, the ox, and the horse; it then pa.s.ses downwards towards the articulation of the elbow, to terminate in a tendon which is inserted into the olecranon process. The portion of this muscle which is next the scapula is covered by the deltoid.

The external head, situated below the long portion, is directed obliquely downwards and backwards. It arises from the curved crest which, from the deltoid impression of the humerus, is directed upward to meet the articular head of the same bone. This crest limiting the musculo-spiral groove superiorly, and the brachialis anticus arising from the whole extent of this groove, the result is that at this level the external head is in relation with the brachialis anticus. From this origin it is directed towards the elbow, to be inserted into the olecranon, either directly or by the medium of the tendon of the long portion. The part of this muscle which arises from the humerus is covered by the deltoid.

As for the internal head (Fig. 76, 4), which, in the superficial layer, is only visible in its inferior part, on the internal aspect of the arm in those animals in which the elbow is free of the lateral wall of the thorax (the dog and the cat, for example), it arises from the internal surface of the humerus, and thence proceeds to be inserted into the olecranon.

The triceps extends the forearm on the arm.

A fourth muscle exists, which veterinary anatomists include in the study of the three portions of the triceps which we have just been discussing, in giving it the name of _small extensor of the forearm_. But, as this muscle is no other than the anconeus, and as, in human anatomy, we describe the latter, according to custom, in connection with the forearm, it is when on the subject of the latter that we will concern ourselves with it. This grouping of muscles cannot fail to give greater clearness to the description of the muscles of these regions.

=The Supplemental or Accessory Muscle of the Latissimus Dorsi= (Fig. 76, 2; Fig. 77, 1).--Because of the relations, to which we have already referred (see p. 142), of this muscle with the triceps cubiti, its description very naturally follows that of the latter.

Indeed, this supplementary muscle of the great dorsal is further designated in zoological anatomy under the name of _long extensor of the forearm_; and this name indicates that its study may be united to that of the triceps.

Situated on the internal surface of the arm, it arises from the external aspect of the tendon of the latissimus dorsi; it is very highly developed in the horse, in which it also arises from the posterior border (axillary) of the scapula; then, covering in part the internal head of the triceps and also the long portion, on the superior border of which it is folded, it proceeds to be inserted into the olecranon process and the anti-brachial aponeurosis.

It extends the forearm on the arm. Further, it makes tense the aponeurosis into which it is inserted; this explains the name of _tensor of the fascia of the forearm_, which is sometimes given to it.

It seems to us interesting to add that, abnormally, we sometimes find in man an a.n.a.logue of this muscle. It is given off from the latissimus dorsi, near the insertion of the latter into the humerus; it accompanies the long head of the triceps and becomes fused with it. Sometimes it is inserted into the olecranon process, at other times into the antibrachial aponeurosis or the epitrochlea. It is on account of its insertion into the last-mentioned, in some cases, that it is also designated by the name of _dorso-epitrochlear_ muscle.[25]

[25] L. Testut, 'Anomalies musculaires chez l'homme expliquees par l'anatomie comparee,' Paris, 1884, p. 118. A. F. Le Double, 'Traite des variations du systeme musculaire de l'homme et de leur signification au point de vue de l'anthropologie zoologique,' Paris, 1897, t. i., p. 203. edouard Cuyer, 'Anomalies musculaires' (_Bulletins de la Societe Anthropologique_, Paris, 1893).

Muscles of the Forearm

Before commencing the special examination of each of the muscles of this region, it is absolutely indispensable to consider their general arrangement, and to determine very clearly how we should study them. We are too well convinced of the importance of this preliminary examination to dismiss it without entering rather fully into it. Indeed, the region on the myological study of which we are now entering is, unquestionably, one of the most complicated with which we have to deal. We know besides, in regard to the study of the forearm in man, how much a definite method is necessary in order that the arrangement of the muscles of this region be fixed in the memory, and that we are unable to obtain this result otherwise than by grouping the twenty muscles which const.i.tute it in clearly defined regions.