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. [Illustration: 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 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.