It acts slightly, however, as a supinator, for, being very oblique downwards and inwards at the time of pronation, 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 pronation 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 passes obliquely downwards and inwards, to be inserted into the inferior part of the internal surface of the radius. It assists 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 passing 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 mass appears in the angular space 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.