The bat and birds have the bones of the forearm so arranged that the radius cannot rotate around the ulna. This is necessary in order that during flight, when the wing is being lowered, the radius and hand shall not be able to turn; for, if such rotation took place, each stroke of the wing would place it in a vertical position, which would occasion a loss of resistance incompatible with the effect to be obtained.
The skeleton of the forearm in quadrupeds is vertical in direction; consequently, it forms with the arm an angle open anteriorly; this is well seen on examining the lateral surface. If we examine it on its anterior surface, we find a slight obliquity directed downwards and inwards. In animals in which the bones of the forearm are separate--that is to say, susceptible of supination and pronation--we find a more close resemblance to those of the human skeleton. The ulna, the superior extremity of which always projects beyond that of the radius, has a shaft which gradually narrows from above downwards. Its inferior extremity is terminated by a round head in those animals in which the ulna is fully developed; in others, as it is atrophied, it ends in a thin, long process.
The ulna presents at its superior extremity a posterior process, the olecranon, which forms the point of the elbow. We find on the anterior surface of the same, another process, the coronoid.
It is necessary to dwell on the relations of these parts. In man the head of the radius is situated at the anterior part of the external surface of the superior extremity of the ulna (Fig. 25); indeed, the small sigmoid cavity with which the head articulates is situated on the outer side of the coronoid process, and this apophysis is placed in front. In the plantigrades and digitigrades the head of the radius is placed still more forward, so much so that it is situated almost in front of the superior extremity of the ulna (Fig. 26). In the unguligrades it is placed directly in front of this latter (Fig. 27).
Further, the displacement of the radius is made at the expense of the superior extremity of the neighbouring bone; the radius appears to appropriate more and more the parts which in man belong exclusively to the ulna--for example, the coronoid process. In the plantigrades and the digitigrades half of the process still belongs to the ulna and the remainder to the radius. In the ungulates--the horse, for example--the coronoid process belongs to the radius; the ulna, situated behind the latter, is correspondingly diminished in size.