204 THE SUPERIOR EXTREMITY 



is situated about 25 mm. (one inch) distal to the tuberosity of 

 the radius. Distally, it blends with the capsule of the distal 

 radio-ulnar joint. The fibres which compose it run for the most 

 part obliquely distally and medially from the radius to the ulna, 

 although several strands may be noticed taking an opposite 

 direction. The dorsal interosseous vessels pass backwards, 

 above its proximal margin, between the two bones of the 

 forearm ; whilst the terminal branch of the volar interosseous 

 artery pierces it about two inches from its distal end. The 

 membrane braces the two bones together in such a manner 

 that forces, passing proximally through the radius, are trans- 

 mitted from the radius to the ulna ; and it extends the surface 

 of origin for the muscles of the forearm. By its volar 

 surface it gives origin to the flexor digitorum profundus and 

 the flexor pollicis longus muscles, whilst from its dorsal 

 surface spring fibres of the two extensor muscles of the 

 thumb, the abductor pollicis longus, and the extensor indicis 

 proprius. 



Chorda Obliqua (O.T. Oblique Ligament). The oblique 

 cord is a weak band of fibres which springs from the tuberosity 

 of the ulna. It extends obliquely, distally and laterally, to 

 find its attachment to the radius, immediately distal to the 

 tuberosity of that bone. It crosses the open space between 

 the bones of the forearm proximal to the proximal border 

 of the interosseous membrane. The oblique cord is often 

 absent ; and unless the utmost care is taken in removing the 

 adjacent muscles it is apt to be injured. 



Movements at the Radio-ulnar Joints. The movements of prona- 

 tion and supination take place at the radio-ulnar joints. When the limb 

 is in a condition of complete supination the thumb is directed laterally, the 

 two bones of the forearm are parallel, the radius lying along the lateral side 

 of the ulna. In the movement of pronation the radius is thrown across the 

 ulna, so that its distal end comes to lie across the volar surface and on the 

 medial side of the ulna. Further, the hand moves with the radius, and 

 when the movement is completed the dorsal aspects of the hand and the 

 forearm are directed forwards, and the thumb is turned medially. 



The dissector should analyse, as far as possible in the part upon which 

 he is engaged, the movements at the two radio-ulnar joints which produce 

 these effects. At the same time it is to be remembered that results 

 obtained from a limb in which the dissection has proceeded so far are apt 

 to be deceptive. Therefore the dissector should use himself and his friends 

 as subjects on which to study the movements. 



In the case of the proximal radio-ulnar joint the movement is simple 

 enough. The head of the radius merely rotates within the annular 

 ligament, and accuracy of motion is obtained by the fovea capituli radii 

 resting and moving upon the distal end of the humerus. It should 



