156 THE UPPER EXTREMITY 



capsule consists of lax fibres which can have little influence 

 in retaining the distal extremities of the bones in apposition. 

 The capsule is attached to the anterior and posterior surfaces 

 of both bones of the forearm, to the lower parts of the inter- 

 osseous crests and to the anterior and posterior borders of the 

 discus articularis. A diverticulum of the capsule which is 

 prolonged from the joint area for some distance proximally, 

 between the bones of the forearm, is called the recessus 

 sacciformis. 



Discus Articularis (O.T. Triangular Fibro-Cartilage). 

 The discus articularis is the true bond of union at the 

 inferior radio-ulnar joint. It has already been noticed in 

 connection with the radio-carpal joint, where it extends 

 the radial articular surface in a medial direction, and is 

 interposed between the lower end of the ulna and the os 

 lunatum. It is a thick, firm plate, attached by its base to 

 the distal margin of the incisura ulnaris of the radius. The 

 apex of the cartilage is directed medially, and is fixed to the 

 depression on the distal end of the ulna at the root of the 

 styloid process. It intervenes between the inferior radio-ulnar 

 joint and the radio-carpal joint. 



Stratum Synoviale (Synovial Membrane). The synovial 

 membrane of the proximal radio-ulnar joint is continuous 

 with that of the elbow-joint. It is prolonged distally so as 

 to line the lig. annulare, and it protrudes beyond this for 

 a short distance upon the neck of the radius. 



The cavity of the inferior radio-ulnar joint and the synovial 

 membrane are prolonged between the head of the ulna and 

 the discus articularis. 



Sometimes the discus articularis is perforated ; when this is the 

 case, the inferior radio-ulnar joint-cavity communicates with the cavity 

 of the radio-carpal joint. 



Membrana Interossea Antibrachii (O.T. Interosseous Mem- 

 brane). This is a fibrous membrane which stretches across 

 the interval between the two bones of the forearm, and is 

 attached to the crista interossea of each. Its proximal border 

 is situated about an inch distal to the tubercle of the radius. 

 Distally it blends with the capsule of the inferior 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 slips may be noticed taking an opposite 

 direction. The dorsal interosseous vessels pass dorsally, 



