300 



HUMAN ANATOMY. 



axis of the ulna. When the radius is brought into semipronation (so that the 

 thumb will point upward) the bones are most nearly parallel and at the greatest 

 possible distance from each other, and the membrane is approximately tense (Fig. 

 315). The forearm is broadest at about the middle. The membrane is at the 

 bottom of a moderate hollow seen from either the front or the back. In extreme 

 supination the anterior hollow is effaced and the posterior deepened. The radius 

 approaches the ulna, especially above the middle. In extreme pronation the front 

 hollow is much deepened and the hind one lost. The bones are much nearer 

 together than in any other position. The radius crosses the ulna, and is above and 

 internal to it at the wrist. 



Should the capsule be opened from below without disturbing the triangular car- 

 tilage in a specimen from which the hand has been disarticulated, in supination the 

 front of the under side of the head of the ulna will be exposed ; in forced |bronation 



Fig. 315. 



Pronation. 



Supination. 



Interosseous membrane^ 



Head of ulna 



r\?s 



-Oblique ligament 



— Interosseous membrane 



Position of the bones of the forearm in pronation and supination. 



almost the whole under end will appear (Figs. 313, 314). As the radius passes 

 behind the head, the ligament of the triangular cartilage is relaxed and the band 

 at the back of the joint is made tense. This ligament becomes tense before com- 

 plete supination and is somewhat relaxed when supination is extreme. 



The motion above described is the only one between the radius and ulna ; 

 nevertheless, in certain movements of twisting the hand and arm the ulna plays a 

 part to be considered later (page 304). 



Surface Anatomy of the Radius and Ulna.— The position of both bones 

 can be felt in a body that is not very muscular, though comparatively little of them 

 is subcutaneous. The triangular space of the back of the olecranon, and the pos- 

 terior border of the ulna starting from it and running to the styloid process, can all 

 be traced with the finger. When the arm is straight, the top of the olecranon is a 

 little above the level of the internal condyle and behind it ; when the arm is bent at 

 a' right angle, the top of the olecranon is in the same vertical plane as the back of 



