RADIO-ULNAR ARTICULATIONS. 



355 



pollicis. Extensor brevis pollicis, Extensor longus pollicis, Extensor indicis ; and, 

 near the wrist, with the anterior interosseous artery and posterior interosseous 

 nerve. 



3. INFERIOR RADIO-ULNAR ARTICULATION. 



This is a pivot-joint, formed by the head of the ulna received into the sigmoid 

 cavity at the inner side of the lower end of the radius. The articular surfaces are 

 covered by a thin layer of cartilage, and connected together by the following lig- 

 aments : 



Anterior Radio-ulnar. Posterior Radio-ulnar. 



Interarticular Fibro-cartilage. 



The Anterior Radio-ulnar Ligament (Fig. 248) is a narrow band of fibres 

 extending from the anterior margin of the sigmoid cavity of the radius to the 

 anterior surface of the head of the ulna. 



The Posterior Radio-ulnar Ligament (Fig. 249) extends between similar points 

 on the posterior surface of the articulation. 



Inferior radio-ulnar 



articulation 



Wrist- joint 



Cafpal articulations. 



Carpo-metacarpal 



articulation 



FIG. 249. Ligaments of wrist and hand. Posterior view. 



The Interarticular Fibro-cartilage (Fig. 251) is triangular in shape, and is 

 placed transversely beneath the head of the ulna, binding the lower end of this 

 bone and the radius firmly together. Its circumference is thicker than its centre, 

 which is thin and occasionally perforated. It is attached by its apex to a depression 

 which separates the styloid process of the ulna from the head of that bone; and 

 by its base, which is thin, to the prominent edge of the radius, which separates the 

 sigmoid cavity from the carpal articulating surface. Its margins are united to the 

 ligaments of the wrist-joint. Its tipper surface, smooth and concave, articulates 

 with the head of the ulna, forming an arthrodial joint ; its under surface, also 

 concave and smooth, forms part of the wrist-joint and articulates with the cuneiform 

 bone. Both surfaces are lined by a synovial membrane the upper surface, by 

 one peculiar to the radio-ulnar articulation ; the under surface, by the synovial 

 membrane of the wrist. 



The Synovial Membrane (Fig. 251) of this articulation has been called, from 

 its extreme looseness, the membrana saccifonnis ; it extends horizontally inward 

 between the head of the ulna and the interarticular fibro-cartilage, and upward 

 between the radius and the ulna, forming here a very loose cul-de-sac. The quan- 

 tity of synovia which it contains is usually considerable. 



Actions. The movement in the inferior radio-ulnar articulation is just the 

 reverse of that between the two bones above. It consists of a movement of rota- 



