[Illustration: FIG. 29.--INFERIOR EXTREMITY OF THE BONES OF THE FOREARM OF THE DOG: LEFT SIDE, ANTERIOR SURFACE, NORMAL POSITION--THAT IS, THE POSITION OF PRONATION.
1, Radius; 2, ulna; A, groove for the long abductor and for the short extensor of the thumb; B, groove for the radials; D, groove for the long extensor of the thumb, the special extensor of the index-finger, and the common extensor of the fingers; E, groove for the special extensor of the little finger.]
We know that on the posterior surface of the inferior extremity of the bones of the human forearm are grooves in which pass the tendons of the posterior and external muscles which, belonging to this region, are directed for insertion towards the hand.
In animals, because of the movement of rotation of the radius, the surface of this bone, which is anterior, corresponds to the posterior surface of the same in man. (To possess a clear conception of this, it is necessary to remember that, in this latter, the bones of the forearm are always described as in the position of supination; they are thus represented in Fig. 28. The direction of the surfaces of the radius is the reverse of that in animals, since the latter have the radius always in a state of pronation.)
Consequently it is on the anterior surface of the bone that we find the grooves concerning which it is necessary to give some details. Regarding them in passing from the radius towards the ulna, those grooves give passage to the tendons of the muscles whose names occupy the columns on p. 43. The letters which are referred to each serve to define their order, and to facilitate reference to Figs. 28, 29, and 30.
[Illustration: FIG. 30.--INFERIOR EXTREMITY OF THE BONE OF THE FOREARM OF THE HORSE: LEFT SIDE, ANTERIOR SURFACE.
1, Radius; A, groove for the long abductor and the short extensor of the thumb; B, groove for the radials; D, groove for the common extensor of the digits; E, groove for the special extensor of the little finger.]
We should mention that the groove E is situated, both in man and in the dog, at the level of the inferior radio-ulnar articulation; but that in the horse, as the ulna does not exist at that level, the groove is situated on the external surface of the inferior extremity of the radius. It is necessary to add that, in some horses, the ulna is, nevertheless, represented in this region by a tongue-like process of bone; and in such cases the groove is situated in front of this process, at the level of the line of coalescence, which there represents the articulation.
