272 THE ARM AND FORE-ARM. [CHAP. 



opposite side is produced into the radial styloid process. 

 The inferior surface is hollowed for articulation with the 

 carpus. The whole bone is slightly curved. Not far below 

 the head is a rough prominence, into which the tendon of 

 the biceps flexor muscle is inserted. 



The ulna has a large sigmoid excavation above for articu- 

 lation with the trochlea of the humerus ; the pointed elevated 

 anterior edge of this is called the coronoid process. The 

 olecranon is scarcely produced upwards beyond the hinder 

 edge of the articular surface. Below this, on the radial side, 

 is a smaller excavation, in which the edge of the disk-like 

 head of the radius plays, being held in its place in the living 

 state by a strong annular ligament, which encircles it. The 

 ulna is straighter than the radius, and gradually diminishes 

 in size to the lower end, where it terminates in a rounded 

 surface, which articulates with the hollow in the lower end 

 of the radius ; and also, though not very directly with the 

 proximal surface of the carpus. The ulnar side is produced 

 into a small conical process, the ulnar styloid process. 



The movement of these bones upon the humerus at the 

 elbow-joint is simply that of a hinge, formed mainly by the 

 articular surface of the ulna. In pronation and supination, 

 which is more free and complete than in any other Mammal, 

 the ulna is stationary, and the radius moves ; the upper end 

 only on its own axis, being fixed to the side of the ulna by 

 the annular ligament, but the lower end rotates round the 

 lower end of the ulna, carrying the hand with it. 



The higher Apes have the axis of the humerus almost as 

 much twisted on itself as in Man ; and they also, almost 

 alone among Mammals, resemble Man in not having the 

 olecranon process of the ulna prolonged upwards beyond 

 the sigmoid notch. Even in the Baboons these special an- 

 thropoid characters are wanting. The humerus has no supra- 



