RADIO-ULNAR JOINTS 203 



of the capsule of the elbow joint and by the radial collateral 

 ligament, which are attached to its proximal border. Its distal 

 border is attached, loosely, to the neck of the radius by a pro- 

 longation of the synovial stratum, which is covered externally 

 by a thin layer of fibrous tissue. 



The Capsule of the Distal Radio -ulnar Joint. This 

 capsule consists of lax fibres which can have little influence 

 in retaining the distal extremities of the bones in apposi- 

 tion. It is attached to the volar and dorsal surfaces of 

 both bones of the forearm, to the most distal parts of the 

 interosseous crests and to the volar and dorsal borders of the 

 articular disc. 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 articular disc is the true bond of union at the distal 

 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 distal end of the ulna and the lunate and 

 triquetral bones. It is a thick, firm plate, attached by its 

 base to the distal margin of the ulnar notch of the radius. 

 The apex of the disc is placed 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 distal radio-ulnar 

 joint and the radio-carpal joint (Fig. 97). 



Stratum Synoviale (Synovial Membrane). The synovial 

 stratum of the proximal radio-ulnar joint is continuous with 

 that of the elbow joint. It is prolonged distally to line the 

 annular ligament, and it protrudes beyond that ligament for 

 a short distance upon the neck of the radius. 



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

 stratum are prolonged between the head of the ulna and 

 the articular disc. 



Sometimes the articular disc is perforated ; when that is the case, 

 the distal radio-ulnar joint-cavity communicates with the cavity of the 

 radio-carpal joint. 



Membrana Interossea Antibrachii. The interosseous 

 membrane 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 



