In the cat, there is a fourth tendon, which pa.s.ses 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 h.o.m.ologue 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 s.p.a.ce, 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 a.s.sists 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.

This muscle flexes the hand on the forearm, and in animals in which the radio-carpal articulation permits, by its formation, it inclines the hand slightly outwards--that is, abducts it.

=Anconeus= (Fig. 72, 17; Fig. 73, 13).--We have already stated (p. 174) that the anconeus is included with the triceps brachialis in zoological anatomy, and that veterinary anatomists give it the name of _small extensor of the forearm_.[28]

[28] It is also called by some authors, the _small anconeus_.

In the dog it recalls, as to position, the human anconeus, but with this difference--that, in the latter, the anconeus, triangular in outline, has one of its angles turned outwards (the epicondyloid attachment) and one of its sides turned towards the olecranon. Here it is entirely the opposite. The anconeus, similarly triangular, is broader externally. At this level it takes its origin from the external border of the humerus, the epicondyle, and the external lateral ligament of the articulation of the elbow; thence its fibres converge towards the external surface of the olecranon, to be there inserted.

It is in relation, anteriorly and inferiorly, with the posterior ulnar muscle. It is covered superiorly by the external head of the triceps. In the cat the disposition of the anconeus is a.n.a.logous. But in the other quadrupeds with which we are here concerned it is completely covered by the external head of the triceps. It really partic.i.p.ates in the formation of the triceps; and seeing that it takes origin from the posterior surface of the humerus at the margin of the olecranon fossa (Fig. 72), and proceeds thence towards the olecranon to be inserted, we can understand why veterinary anatomists have connected its study with that of the posterior muscular ma.s.s of the arm.

This muscle is an extensor of the forearm on the arm.

We proceed now to inquire what the deep muscles of the posterior region of the human forearm become in quadrupeds: the long abductor of the thumb, the short extensor of the thumb, the long extensor of the thumb, the proper extensor of the index. We know that in every instance these muscles, which are deeply seated at their origin, become superficial afterwards.

In quadrupeds, on account of the position in which the forearm is placed--viz., p.r.o.nation--the corresponding muscles occupy the anterior aspect of this region.

=Long Abductor of the Thumb= (_Extensor ossis metacarpi pollicis_) =and Short Extensor of the Thumb= (_Extensor primi internodii pollicis_) (Fig. 73, 14; Fig. 74, 19; Fig. 75, 20).--United one to the other in man, blended in quadrupeds, they form in the latter the muscles to which veterinary anatomists give the name of _oblique extensor of the metacarpus_.

This muscle arises from the median portion of the skeleton of the forearm. There it is covered by the common extensor of the digits and that of the small digit (anterior extensor and lateral extensor of the phalanges). Then, at the internal border of the first of these muscles, it becomes superficial, pa.s.ses downwards and inwards, crosses superficially the anterior extensor of the metacarpus, reaches the inferior extremity of the radius, and becomes lodged in the most internal of the grooves situated on the anterior surface of this extremity, pa.s.ses on the internal side of the carpus, and is inserted into the superior extremity of the most internal metacarpal--that is, to the first metacarpal, or metacarpal of the thumb--in the dog and cat; to the internal rudimentary metacarpal in the horse.

It is an extensor of the metacarpal into which it is inserted; but as, if we recall the extreme examples given above, in the dog the first metacarpal is not very mobile, and in the horse the internal rudimentary metacarpal is absolutely fixed to the bone which it accompanies, it is more exact to add that this muscle is princ.i.p.ally an extensor of the metacarpus as a whole.

And yet, in the cat and the dog, it is also able to adduct the first metacarpal bone. It must be understood that this movement would be abduction, if the hand could be placed in the position of complete supination, as in the human species.

=Long Extensor of the Thumb= (_Extensor secundi internodii pollicis_) =and Proper Extensor of the Index= (_Extensor indicis_).--These two muscles are blended together by their fleshy bodies, so that the single name of _proper extensor of the thumb and index_ is preferable. This muscle is but of slight importance from our point of view, for it is extremely atrophied, and so much the more as the number of the digits is lessened.

It arises, as the preceding, from the skeleton of the forearm, and there it is deeply placed. Below, towards the carpus, its tendinous part becomes superficial, to end in the following manner:

In the carnivora, the tendon divides into two very slender parts, which are inserted into the thumb and the index. In the pig, the tendon is blended with that of the common extensor of the internal digits.

Finally, in the ox and the horse, it is sometimes regarded as being blended with the common or anterior extensor of the phalanges. But to us it appears more rational to say that it does not exist, which, moreover, is explained by the digital simplification of the hand.

Internal and Posterior Region

=p.r.o.nator Teres= (Fig. 76, 8).--This muscle, as may easily be understood, undergoes, as do the supinators, a degree of degeneration in proportion to the loss of mobility of the radius on the ulna. In animals in which the bones of the forearm are not fused it exists; in those, on the other hand, in which this segment has become simply a supporting column, it is not developed--at least, in a normal manner.

It is, consequently, found best marked in the dog and the cat.

Forming, as in man, the internal limit of the hollow of the elbow, the p.r.o.nator teres has a disposition a.n.a.logous to that which characterizes the corresponding muscle in the human species. It arises from the epitrochlea (internal condyle), proceeds downwards and outwards, and is inserted into the middle portion of the body of the radius.

It is into the hollow in front of the elbow, which this muscle contributes to limit, that the biceps and the brachialis anticus dip.

In the pig and the ox it is atrophied.

In the horse it does not exist. We may, however, sometimes find it, but in an abnormal form. We were able to demonstrate its presence in the form of a fleshy tongue situated on the internal side of the elbow (Fig. 78) in a horse which we dissected many years ago in the laboratory of the School of Fine Arts. Moreover--and the fact seemed to us an interesting one--the forearm to which the muscle belonged had an ulna of relatively considerable development (Figs. 79 and 80).[29]

[29] edouard Cuyer, 'Abnormal Length of the Ulna and Presence of a p.r.o.nator Teres Muscle in a Horse' (_Bulletin de la Societe d'Anthropologie_, Paris, 1887).

This muscle is a p.r.o.nator.

=Flexor Carpi Radialis= (Fig. 76, 10; Fig. 77, 7).--Called by veterinary anatomists _the internal flexor of the metacarpus_, this muscle, which is found on the internal aspect of the forearm, is situated behind the p.r.o.nator teres when this muscle exists, whilst in the animals which are deprived of the latter the flexor carpi radialis has in front of it the internal border of the radius, which separates it from the anterior extensor of the metacarpus.

It is necessary to add that the flexor carpi radialis is similarly separated from the anterior extensor of the metacarpus by the internal border of the radius in animals in which the p.r.o.nator teres exists, but then only in that part of the forearm which is situated below this latter.

The flexor carpi radialis arises from the epitrochlea. Its fleshy body, fusiform in shape, descends vertically, and terminates in a tendon on the posterior surface of the bases of the second and third metacarpals in the dog and the cat, on the metacarpal of the large internal digit in the pig, on the internal side of the metacarpus in the ox, and on the superior extremity of the internal rudimentary metacarpal in the horse.

We see clearly, in this latter, a superficial vein which, in the shape of a strong cord, pa.s.ses along the anterior border of the flexor carpi radialis; it is the subcutaneous median or internal vein, which, forming the continuation of the internal metacarpal vein, joins the venous system of the arm, after having crossed obliquely the corresponding part of the radius.

=Palmaris Longus.=--This muscle, which exists distinctly in some animals, but whose absence is far from being rare in the human species, is not developed as a distinct muscle in any of the domestic quadrupeds.

[Ill.u.s.tration: FIG. 76.--MYOLOGY OF THE DOG: LEFT ANTERIOR LIMB, INTERNAL ASPECT.

1, Biceps; 2, long extensor of the forearm (supplementary or accessory muscle of the great dorsal); 3, triceps, long head; 4, triceps, internal head; 5, olecranon; 6, epitrochlea (internal condyle); 7, radial extensors (anterior extensor of the metacarpus); 8, p.r.o.nator teres; 9, radius; 10, flexor carpi radialis (internal flexor of the metacarpus); 11, anterior ulnar (oblique flexor of the metacarpus); 12, superficial flexor of the digits; 13, deep flexor of the digits; 14, flexor longus pollicis (radial fasciculus of the deep flexor of the digits); 15, pisiform bone.]

And yet some authors announce its presence in the dog, and describe it as becoming detached, in the form of a cylindrical bundle, from the anterior surface of the fleshy ma.s.s of the deep flexor of the digits (see p. 196) to proceed then by a tendon which divides into two parts, to terminate in the palm of the hand, where it blends with the tendons of the superficial flexor, which are destined for the third and fourth digits.

These authors give to this muscle the name of _palmaris longus_, and attribute to it the action of flexing the hand.

=Anterior Ulnar= (_Flexor carpi ulnaris_) (Fig. 73, 16; Fig. 76, 11; Fig. 77, 8).--Called by veterinary anatomists the _oblique flexor of the metacarpus_, or _internal ulnar_, this muscle occupies the internal part of the posterior aspect of the forearm in the ox and the horse, while in the dog it occupies rather the external part.

This difference arises from the fact that in this latter, as in man, the anterior ulnar is separated from the flexor carpi radialis by an interval in which we see, on the internal aspect of the forearm, just at the level of the elbow, the flexors of the digits. This interval is so much the wider as there is no palmaris muscle to subdivide its extent (Fig. 81). In the horse, the interval in question does not exist. In this animal, indeed, the anterior ulnar is in contact with the radial flexor, so that this muscle can occupy only a region belonging rather to the internal surface of the forearm (Fig. 82).

In the dog the anterior ulnar is in contact with the posterior ulnar.

This relation recalls that which is found in man, where the two muscles are merely separated by the crest of the ulna (Fig. 81). But in the horse, in which the anterior ulnar has, so to speak, slid towards the internal aspect, this muscle is separated above from the posterior ulnar, and it is in the interval separating these two muscles that we are able to perceive, but this time at the back of the forearm, the muscular ma.s.s of the flexors of the digits (Fig. 82).

[Ill.u.s.tration: FIG. 77.--MYOLOGY OF THE HORSE: ANTERIOR LIMB, LEFT SIDE, INTERNAL ASPECT.

1, Long extensor of the forearm (supplementary or accessory muscle of the latissimus dorsi); 2, radialis muscles (anterior extensor of the metacarpus); 3, tendons of extensor ossis metacarpi pollicis extensor primi internodii pollicis united (oblique extensor of the metacarpus); 4, tendon of extensor communis digitorum (anterior extensor of the phalanges); 5, strengthening band from the suspensory ligament of the fetlock; 6, internal surface of the radius; 7, flexor carpi radialis (internal flexor of the metacarpus); 8, anterior ulnar (oblique flexor of the metacarpus); 9, pisiform bone; 10, 10, tendon of the superficial flexor of the digits (superficial flexor of the phalanges); 11, 11, tendon of the deep flexor of the digits (deep flexor of the phalanges); 12, sesamoid prominence; 13, suspensory ligament of the fetlock; 14, internal rudimentary metacarpal.]