REGION OF THE WRIST. 331 



Of the bones of the forearm the radius and ulna we have seen that at the 

 elbow the ulna is the larger of the two. This is because the main function of the 

 ulna is to act as a support to the parts beyond. The radius is intended mainly as a 

 means of enabling the hand to perform the functions of pronation and supination. 

 At the wrist we find the radius supporting the hand and consequently its lower 

 end is large and well developed. The ulna, on the contrary, contributes but little to 

 the support of the hand and does not even enter directly into the wrist-joint, as does 

 the radius at the elbow-joint, but serves as a fixed point around which the radius 

 rotates. The functional value of the ulna at the wrist is so much less than that of 

 the radius as amply to account for its diminished size. 



Posterior radial or thecal tu 



Ulna 



Head of ulna 



Styloid process of ulna 

 Semilunar 



Cuneiform 



Os magnum 

 Unciform 



Metacarpal bone of the th 



FIG. 340. Posterior view of the lower end of the radius and ulna and the carpal bones. 



Lower end of the Radius. The lower end of the radius is large and spongy. 

 The compact tissue forms a quite thin superficial layer (Fig. 338). Its anterior 

 surface is hollowed out to receive the pronator quadratus muscle, with a prominent 

 articular edge to which is attached the anterior ligament (Fig. 339). 



The posterior surface is convex and marked with a number of ridges with 

 grooves between them which lodge the extensor tendons (Fig. 340). In its middle 

 is a prominence, the dorsal radial tubercle, which marks the position of the extensor 

 longus pollicis muscle. On its inner side is a concave articular facet, the ulnar notch 

 (sigmoid cavity}, for articulation with the ulna; it is plane from above downward, 

 thus showing that it permits movement in one direction only, like a hinge. 



Between the lower edge of the ulnar notch and the articular surface is a rough 

 ridge that gives attachment to the triangular interarticular fibrocartilage. 



