Chapter II: Myology (2)

=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 vertebrae; 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 vertebrae; thence it proceeds to be inserted into the mastoid crest, and the transverse processes of the atlas and three or four vertebrae 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 scapulae. 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 aesthetic 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 scapulae. 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.

1, Cartilage of prolongation of the scapula; 2, tuberosity of the spine of the scapula; 3, superior extremity of the humerus; 4, inferior extremity of the humerus; 5, radius; 6, ulna; 7, supraspinatus muscle; 8, infraspinatus; 9, teres minor; 10, biceps; 11, tendon of the biceps passing over the anterior surface of the superior extremity of the humerus; 12, brachialis anticus; 13, triceps, long head; 14, external head of the triceps divided; 15, external head of the triceps reflected, in order to expose the anconeus; 16, region normally occupied by the external head of the triceps; 17, anconeus.]

=Teres Minor= (Fig. 72, 9).--This muscle, also called in veterinary anatomy _the short abductor of the arm_, arises from the posterior border of the scapula (the external border in man), and is inserted below the great tuberosity of the humerus, between the attachments of the infraspinatus and deltoid.

It is covered by the deltoid and the infraspinatus.

=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 passes to be inserted into the internal surface of the humerus.

It is covered by the latissimus dorsi and the posterior muscular mass 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 mass 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 constituted by a single muscle, the triceps.

In animals, we find them in the same number and arranged in analogous fashion--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 mass of the arm, it does not undergo the same change. The muscle which constitutes it--the triceps cubiti--occupies, on the contrary, a greater area. Let us suppose, further--for it is the best method of comprehending the homologies 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 analogous 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 analogy 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 passing downwards and inwards, as does the latter, it terminates on the muscular mass which constitutes 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, passes 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 space 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 interspace 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 attitude that the region which it occupies is accessible to view.

But an analogous attitude 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 passes 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 space 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 mass, that if the deltoid does not constitute in quadrupeds a prominence sufficient to remind one of that which this muscle produces in man, the triceps, in producing an analogous 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 passes 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 analogue 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 constitute it in clearly defined regions.

We also know that these muscles are first studied with the forearm in the position of supination, and that it is only when they are well known after having considered them in this position that we are able to analyze and comprehend their forms when it is in pronation.

Now, as we have pointed out in the section on osteology (see p. 34), the forearm in quadrupeds is always in the position of pronation. Should we, then, in order to maintain the symmetry with human anatomy, first study the forearm in the position of supination? Evidently not. Besides the fact that this would in some cases be impossible since--as in the horse, for example--the radius and ulna are fused together, we should not gain any advantage; this position being never completely realizable even in those quadrupeds which have the radius relatively movable--as, for example, in the cat.

Accordingly, it is pronation which here, in connection with animals, becomes the standard attitude from the point of view of description. This is why, supposing that the reader knows well the muscles of the human forearm in the position of supination, we should recall what is the general arrangement occupied by these muscles when it is in pronation.

The fore-limb, being viewed on its anterior surface, presents above the anterior aspect of the region of the elbow; but below, it is the posterior surface of the wrist which is seen. Consequently, in the superior part, we see the external and anterior muscles limiting the hollow in front of the elbow; interiorly are found the posterior muscles.

The long supinator, passing obliquely downwards and inwards, divides, in fact, the forearm into two parts: one supero-internal, the other infero-external. In the first we see, but to an extent less and less considerable, the pronator teres, the flexor carpi radialis, the palmaris longus, and the flexor ulnaris; as to the flexors of the digits, on account of the rotation of the radius, they are only visible on the opposite surface--that is to say, on the surface of the wrist, which is now posterior. In the second part we see the two radial extensors, the common extensor of the fingers, the proper extensor of the little finger, and the ulnar extensor which, inferiorly, remains behind, by reason of the position of the ulna being unchanged, whilst the anconeus is wholly posterior, since the direction of the elbow is not modified. We also find, in this region, the long abductor of the thumb, the short extensor of the thumb, the long extensor of the thumb, and the special extensor of the index-finger, in the region where these deep muscles become superficial.

So that, to summarize, the external and posterior muscles occupy the anterior and external regions of the forearm, whilst the anterior muscles occupy rather the internal and posterior. It is in regarding them after this manner--that is to say, arranged in these two regions--that we proceed to study these muscles in quadrupeds.

Anterior and External Region

=Supinator Longus.=--We know that this muscle, which is especially a flexor of the forearm on the arm, plays, notwithstanding the name which has been given it, a part of but little importance in the movement of supination.

It acts slightly, however, as a supinator, for, being very oblique downwards and inwards at the time of pronation, it is able, while tending to resume its vertical direction, to carry the radius outwards; it places, in fact, the forearm in a position midway between pronation and supination.

We have just recalled these details, in order that it may be more easy to understand why it does not exist in animals in which the radius and ulna are fused together (horse, ox); and why, on the other hand, we find traces of it in the cat and the dog, in which the radius--to a slight extent, it is true--is able to rotate on the ulna. This displacement being a little more considerable in the felide, the long supinator is a little further developed than it is in the canine species; but, notwithstanding, it is only rudimentary.

The long supinator arises, above, from the external border of the humerus; thence, in the form of a very narrow fleshy band, it passes obliquely downwards and inwards, to be inserted into the inferior part of the internal surface of the radius.

It assists in turning the radius outwards and placing it in front of the ulna, the movement of supination being capable of being but little further extended.

=First and Second External Radial Muscles=: _Extensor carpi radialis longior and brevior_ (Fig. 73, 8; Fig. 74, 8, 9; Fig. 75, 8, 9).--Fused together, these muscles form by their union what veterinary anatomists call _the anterior extensor of the metacarpus_. But we should add that these two muscles are united so much the more intimately as we examine them in passing successively from the cat to the dog, pig, ox, and horse. Thus, in the cat they are often distinct; in the dog, they unite only at the level of the middle third of the radius, and interiorly they have two tendons; in the pig, the ox, and the horse they are completely united, and there exists but a single tendon.

The _anterior extensor of the metacarpus_, which is situated behind the long supinator when the latter exists, occupies the external aspect of the forearm; its well-defined form absolutely recalls the prominence on the superior part of the external margin of the human forearm.

It arises superiorly from the portion of the external border of the humerus which is situated above the epicondyle and behind the musculo-spiral groove. Its fleshy mass appears in the angular space bounded by the brachialis anticus and the triceps. The superior portion is covered by the external head of the triceps; yet, in the dog, the superior portion of its humeral attachment is the only part so covered. This muscle is directed forward and downwards; it is also inclined a little inwards in such manner as to proceed to occupy the anterior aspect of the forearm.

Its fleshy belly is narrowed below, and, towards the inferior part of the forearm, is continued by a tendinous portion which is situated on the anterior surface of the carpus, after having traversed the median groove of the inferior extremity of the radius.

In the cat and the dog, in which the union of the two radial extensors is incomplete, the two tendons are inserted into the front of the base of the second and third metacarpal bones; consequently, as in man, into the metacarpals of the index and middle fingers.

In the ox, the tendon, which is single, is inserted into the internal and anterior half of the superior extremity of the principal metacarpal.

In the pig, this tendon is attached to the base of the large internal metacarpal.

In the horse, the corresponding tendon is attached to a tubercle which is situated on the anterior surface of the base of the principal metacarpal, a little internal to the median plane of the latter.

1, Mastoido-humeralis; 2, biceps; 3, brachialis anticus; 4, triceps, long portion; 5, triceps, external head; 6, olecranon process; 7, epicondyle; 8, radialis muscles (anterior extensor of the metacarpus); 9, extensor communis digitorum (anterior extensor of the phalanges); 10, extensor minimi digiti (lateral extensor of the phalanges, or common extensor of the three external digits); 11, posterior ulnar (external flexor of the metacarpus); 12, pisiform bone; 13, anconeus; 14, extensor ossis metacarpi pollicis and extensor primi internodii pollicis (oblique extensor of the metacarpus); 15, radius; 16, anterior ulnar (oblique flexor of the metacarpus); 17, external border of the hypothenar eminence (abductor of the little finger).]

In order to properly understand and remember the respective positions occupied by these inferior insertions, it must be remembered that the human forearm being in the position of pronation, the tendons of the radials are attached to the bases of the metacarpals nearest to the thumb--that is to say, those occupying an internal position as regards the fourth and fifth metacarpals.

As its name indicates, this muscle extends the metacarpus. Consequently it is, in the horse, an extensor of the canon-bone.

It is also an adductor of the hand in those animals (cat, dog) in which the radio-carpal articulation, analogous in form to the corresponding articulation in man, permits lateral movements of the hand on the forearm. The union of the fleshy bodies of the two radials is sometimes found in the human species.

=Supinator Brevis.=--As in the case of the long supinator, the short supinator is found only in animals in which the radius can be rotated to a greater or less extent around the ulna; therefore this muscle is not found in the pig, the ox, or the horse; but it forms part of the forearm of the cat and the dog.

Deeply situated at the region of the elbow, the short supinator has little interest for us. All that we will say of it is that it goes from the external part of the inferior extremity of the humerus to the superior part of the radius; and that it is, in carnivora, the essential agent in the production of the movement of supination.

=Extensor Communis Digitorum= (Fig. 73, 9, 10, 11; Fig. 74, 10, 11, 12).--Also named in veterinary anatomy the _anterior extensor of the phalanges_, this muscle is situated external to and behind the anterior extensor of the metacarpus already described.

In the human being, the common extensor of the fingers springs, in its superior part, from the bottom of a depression, situated on the outer side of and behind the elbow, and limited in front by the muscular prominence which the long supinator and the first radial extensor form at that level. At the bottom of this hollow or fossette is found the epicondyle, which gives origin, amongst other muscles, to the common extensor of the fingers. It is necessary to add that it is most prominently visible during supination, and that it tends to be effaced during pronation.

1, Mastoido-humeralis; 2, pectoralis major; 3, deltoid; 4, brachialis anticus; 5, triceps; 6, triceps, external head; 7, olecranon; 8, radial extensors (anterior extensor of the metacarpus); 9, insertion of the tendon of the anterior extensor of the metacarpus to the tubercle of the superior extremity of the principal metacarpal; 10, 11, extensor communis digitorum (10, proper extensor of the inner digits; 11, common extensor of the two digits); 12, tendon of the common extensor of the two digits; 13, band of reinforcement from the suspensory ligament of the fetlock; 14, external tuberosity of the superior extremity of the radius; 15, extensor minimi digiti (proper extensor of the external digit); 16, tendon of the proper extensor of the external digit; 17, posterior ulnar (external flexor of the metacarpus); 18, pisiform; 19, extensor ossis metacarpi pollicis and extensor primi internodii pollicis (oblique extensor of the metacarpus); 20, ulnar portion of the deep flexor of the toes; 21, tendon of the superficial flexor of the toes (superficial flexor of the phalanges); 22, tendon of the deep flexor of the toes (deep flexor of the phalanges); 23, suspensory ligament of the fetlock.]

An analogous arrangement is met with in animals. But the muscular prominence is formed by the united radial extensors, and the fossette, because of the permanent pronation of the forearm, is scarcely recognisable. Likewise, with regard to the dog, we may say that it does not exist, on account of the prominence which the epicondyle forms in that animal (Fig. 73, 7).

In connection with this prominence of the epicondyle, it is interesting to add that this detail recalls the relief which the same process produces on the external aspect of the human elbow when the forearm is flexed on the arm. We know that, in this case, the epicondyle is exposed, because the muscles which mask it in supination (long supinator and long radial extensor) are displaced and set it free during flexion. But, in the dog, as in other quadrupeds besides, the forearm is, in the normal state, flexed on the arm; the latter being oblique downwards and backwards, and the former being vertical. Further, the epicondyle is well developed.

The muscle with which we are now occupied, long and vertical in direction, arises from the inferior part of the external border of the humerus (there it is covered by the anterior extensor of the metacarpus, from which it is freed a little lower down) and from the external and superior tuberosity of the radius. In the carnivora, it arises from the epicondyle. Its fleshy body is fusiform in shape, becomes tendinous in the lower half of the forearm, and then divides into a number of slips, varying in number according to the species; this division is correlated to that of the hand--that is to say, with the number of the digits. Before reaching this latter, the common extensor of the digits passes through the most external groove on the anterior surface of the inferior extremity of the radius.

1, Mastoido-humeral; 2, pectoralis major; 3, deltoid; 4, brachialis anticus; 5, triceps, long head; 6, triceps, external head; 7, olecranon; 8, radial extensors (anterior extensor of the metacarpus); 9, insertion of the tendon of the anterior extensor of the metacarpus into the tubercle of the superior extremity of the principal metacarpal; 10, extensor communis digitorum (anterior extensor of the phalanges); 11, tendon of the anterior extensor of the phalanges; 12, reinforcing band arising from the suspensory ligament of the fetlock; 13, external tuberosity of the superior extremity of the radius; 14, extensor minimi digiti (lateral extensor of the phalanges); 15, tendon of the lateral extensor of the phalanges; 16, fibrous band which this latter receives from the tendon of the anterior extensor of the phalanges; 17, fibrous band which the same tendon receives from the carpal region; 18, posterior ulnar (external flexor of the metacarpus); 19, pisiform; 20, extensor ossis metacarpi pollicis and extensor primi internodii pollicis (oblique flexor of the metacarpus); 21, ulnar portion of the deep flexor of the digits; 22, flexor digitorum profundus; 23, 23, tendon of the flexor digitorum sublimis (superficial flexor of the phalanges); 24, 24, tendon of the flexor digitorum profundus (deep flexor of the phalanges); 25, sesamoid prominence; 26, suspensory ligament of the fetlock; 27, external rudimentary metacarpal.]

In the cat and the dog, the four tendons which result from the division of the principal tendon go to the four last digits, and each of them is inserted, as in the human species, to the second and third phalanges.

In the pig, the anterior extensor of the phalanges is rather complicated in its arrangement. Its fleshy body is divided into four bundles terminated by tendons, which in turn divide and join certain digits; whence the special names given to each of these fasciculi, commencing with the most internal, of: _proper extensor of the great inner toe_; _common extensor of the two inner toes_; _common extensor of the two outer toes_; and _proper extensor of the great outer toe_.

In the ox, the same muscle is divided into two bundles: the internal proceeds to the internal toe, the external is common to the two toes.

In the horse, the tendon of the anterior extensor of the phalanges is divided into two parts of unequal bulk. The smaller of these tendinous slips, which is the more external, unites at the level of the superior part of the metacarpus with the tendon of the muscle which we are about to study in the following paragraph (Fig. 75, 16). The larger, after having reached the anterior surface of the digit, is attached to the anterior aspect of the first and second phalanges, and then forms a terminal expansion which is inserted into the pyramidal eminence of the third.

At the level of the first phalanx this tendon receives on each of its lateral aspects a strengthening band, which proceeds from the terminal extremity of _the suspensory ligament of the fetlock_,[26] and crosses obliquely downwards and forwards over the surface of the first phalanx to join the extensor tendon (Fig. 75, 12).

[26] See p. 200 for a description of this ligament.

A similar arrangement is found in the ox.

This band is noticeable under the skin which covers the lateral aspects of the ham.

As the name indicates, this muscle extends the phalanges, one upon the other. It also contributes to the extension of the hand, as a whole, on the forearm.

=Extensor Minimi Digiti= (Fig. 73, 10; Fig. 74, 15, 16; Fig. 75, 14, 15).--This muscle, _the lateral extensor of the phalanges_ of veterinary anatomy, situated on the external surface of the forearm, behind the common extensor of the digits, arises, as a rule, from the epicondyle (dog, cat), or from the external surface of the superior extremity of the radius (horse). The tendon succeeding to the fleshy body appears towards the lower third of the forearm, and at the level of the wrist lies in a groove analogous to that which in man is hollowed out for the passage of the corresponding tendon at the level of the inferior radio-ulnar articulation. This groove corresponds to the same articulation in animals in which the ulna is well developed, such as the dog and the cat; but it belongs to the radius when the inferior extremity of the ulna does not exist--for example, in the horse. Indeed, in this animal the groove in question is found on the external surface of the carpal extremity of the radius.

In the dog, the tendon is divided into three parts, which, crossing obliquely the tendons of the common extensor of the digits, pass to the three external digits, to be inserted by blending with the corresponding tendons of the latter into the third phalanges of those digits.

Thus is explained the name of _common extensor of the three external digits_ which is sometimes given to this muscle.

In the cat, there is a fourth tendon, which passes to the index-finger, so that the name _common extensor of the four external digits_ is in this case legitimate, and the lateral extensor of the phalanges is also a common extensor, as is the anterior extensor of the phalanges, or common extensor of the digits.

In the pig, the tendon, which is single, is inserted into the external digit, for which reason it has received the name of the _proper extensor of the small external digit_. This muscle is, then, really the homologue of that which exists in the human species.

In the ox, it is called the _proper extensor of the external digit_; it is as thick as the common extensor.

Finally, in the horse, the muscle is little developed. Its fleshy body, thin and flattened from before backwards, becomes distinctly visible only below the middle of the forearm. Above, it is enclosed in a limited space, bounded in front by the common extensor of the digits, and behind by the posterior ulnar; there these two muscles approach each other so closely that from the point of view of external form they seem to be nearly in contact.

The tendon, after receiving the small fasciculus from the common extensor (Fig. 75, 16), as well as a fibrous band emanating from the external surface of the carpus (Fig. 75, 17), is situated at the external side of the tendon of the anterior extensor of the phalanges, and is inserted into the anterior surface of the superior extremity of the first phalanx.

This muscle extends the digit or digits into which it is inserted. It also assists in the movement of extension of the hand as a whole.

=Posterior Ulnar= (_Extensor carpi ulnaris_) (Fig. 73, 11; Fig. 74, 17; Fig. 75, 18).--Designated by veterinary anatomists as the _external flexor of the metacarpus_,[27] or _external cubital_, this muscle is situated in the posterior region of the external surface of the forearm, behind the lateral extensor of the phalanges.

[27] Certain authors give it the name of _ulnar extensor of the
wrist_. It is true that in the human being this is its action;
but in quadrupeds, owing to its insertion into the pisiform, it
draws the hand into the position of flexion.

It arises from the epicondyle; its fleshy body, thick but flattened, is directed vertically towards the carpus, and its tendon is inserted into the external part of the superior extremity of the metacarpus, after having given off a fibrous band, which takes its attachment on the pisiform.

It is inserted, in the cat and the dog, into the superior extremity of the fifth metacarpal; in the pig to the external metacarpal; in the ox to the external side of the canon-bone; in the horse to the superior extremity of the external rudimentary metacarpal.