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. 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 principally an extensor of the metacarpus as a whole.