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 a.n.a.lyze and comprehend their forms when it is in p.r.o.nation.
Now, as we have pointed out in the section on osteology (see p. 34), the forearm in quadrupeds is always in the position of p.r.o.nation. 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 p.r.o.nation which here, in connection with animals, becomes the standard att.i.tude 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 p.r.o.nation.
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, pa.s.sing 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 p.r.o.nator 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 p.r.o.nation, 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 p.r.o.nation 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 pa.s.ses obliquely downwards and inwards, to be inserted into the inferior part of the internal surface of the radius.
It a.s.sists 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 pa.s.sing 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 ma.s.s appears in the angular s.p.a.ce 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 princ.i.p.al 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 princ.i.p.al metacarpal, a little internal to the median plane of the latter.
[Ill.u.s.tration: FIG. 73.--MYOLOGY OF THE DOG: LEFT ANTERIOR LIMB, EXTERNAL ASPECT.
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 p.r.o.nation, 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, a.n.a.logous 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 p.r.o.nation.
[Ill.u.s.tration: FIG. 74.--MYOLOGY OF THE OX: LEFT ANTERIOR LIMB, EXTERNAL ASPECT.
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 princ.i.p.al 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 a.n.a.logous arrangement is met with in animals. But the muscular prominence is formed by the united radial extensors, and the fossette, because of the permanent p.r.o.nation 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 pa.s.ses through the most external groove on the anterior surface of the inferior extremity of the radius.
[Ill.u.s.tration: FIG. 75.--MYOLOGY OF THE HORSE: LEFT ANTERIOR LIMB, EXTERNAL ASPECT.
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 princ.i.p.al 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 princ.i.p.al 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 a.n.a.logous to that which in man is hollowed out for the pa.s.sage 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, pa.s.s 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.