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 analogous. But in the other quadrupeds with which we are here concerned it is completely covered by the external head of the triceps. It really participates 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 mass 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., pronation—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, passes 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, passes 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.