RADTO-ULNAR JOINTS. 



295 



Fig. 91. 



VIEW OP THE ORBICULAR LIGAMENT 

 (a) OF THE RADIUS, WHICH RETAINS 

 TH K UPPER KND OF THE BONK AGAIXST 

 THB ULNA. 



The interosseous membrane (fig. 90, 3 ) is a thin fibrous layer, which is 

 attached to the contiguous margins of the radius and ulna, and forms an 

 incomplete septum between, the muscles on the front and back of the fore- 

 arm. Most of its fibres are directed obliquely inwards towards the ulna, 

 though a few on the posterior surface have 

 an opposite direction. Superiorly the 

 membrane is wanting for a considerable 

 space, and through the interval the poste- 

 rior interosseous vessels pass backwards. 

 Some small apertures exist in it for the 

 passage of vessels ; and the largest of these 

 (*) is about two inches from the lower 

 end, through which the anterior interos- 

 seous artery turns to the back of the wrist. 

 The membrane gives attachment to the 

 deep muscles. 



The round ligament (fig. 90, 2 ) is a 

 slender band above the interosseous mem- 

 brane, whose fibres have a direction oppo- 

 site to those of the membrane. By one 

 end it is fixed to the front of the coronoid 

 process, and by the other to the radius be- 

 low the tubercle. The ligament divides 

 the space above the interosseous membrane 

 into two parts. Oftentimes this band is 

 not to be recognized. 



The lower radio-ulnar articulation cannot be well seen till after the 

 examination of the wrist joint (p. 297). 



Movement of the radius. The radius moves forwards and backwards 

 around the ulna. The forward motion, directing the palm of the hand 

 towards the ground, is called pronation ; and the backward, by which the 

 palm of the hand is placed upwards, is named supination. 



In pronation the upper end of the bone rotates within the band of the 

 orbicular ligament without shifting its position to the ulna. The lower 

 end, on the contrary, moves over the ulna from the outer to the inner 

 side, describing half a circle ; and the shaft crosses obliquely that of the 

 ulna. 



In supination the lower end of the radius turns backwards over the 

 ulna ; the shafts come to be placed side by side, the radius being external ; 

 and the upper end rotates from within out in its circular band. 



In these movements the radius revolves round a line, internal to the 

 shaft, which is prolonged upwards through the neck and head of the bone, 

 and downwards through the centre of a circle of which the small sigmoid 

 cavity of the ulna is a segment (Ward). 



The upper end of the bone is kept in place by the orbicular ligament ; 

 the lower end by the triangular fibro-oartilage ; and the shafts are united 

 by the interosseous ligament, which is tightened in supination, and is re- 

 laxed in pronation. 



In fracture of either bone the movements cease ; in the one case be- 

 cause the radius cannot be moved except it is entire ; and in the other, 

 because the ulnar support is wanting for the revolving radius. 



Dissection. To see the ligaments of the wrist-joint, the tendons and 

 the annular ligaments must be removed from both the front and back ; and 



