312 THE ARTICULATIONS, OR JOINTS 



interosseous ligament, and it appears to be placed as a substitute for it in the 

 upper part of the interosseous interval. This ligament is sometimes wanting. 

 The interosseous membrane (membrana interossea antebrachii) (Fig. 252) is a 

 broad and thin plane of fibrous tissue descending obliquely downward and inward, 

 from the interosseous ridge on the radius to that on the ulna. It is deficient 

 above, commencing about an inch beneath the tubercle of the radius; is broader 

 in the middle than at either extremity; and presents an oval aperture just above 

 its lower margin for the passage of the anterior interosseous vessels to the back of 

 the forearm. This ligament serves to connect the bones and to increase the extent 

 of surface for the attachment of the deep muscles. Between its upper border 

 and the oblique ligament an interval exists through which the posterior inter- 

 osseous vessels pass to the dorsum of the forearm. Two or three fibrous bands 

 are occasionally found on the dorsal surface of this membrane which descend 

 obliquely from the ulna toward the radius, and which have consequently a direc- 

 tion contrary to that of the other fibres. It is in relation, in front, by its upper 

 three-fourths with the Flexor longus pollicis on the outer side, and with the 

 Flexor profundus digitorum on the inner, lying upon the interval between which 

 are the anterior interosseous vessels and nerve; by its lower fourth, with the Pro- 

 nator quadratus; behind, with the Supinator [brevis], Extensor ossis metacarpi 

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

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

 nerve. 



3. INFERIOR ARTICULATION (ARTICULATIO RADIOULNARIS DISTALIS). 



This is a pivot-joint, formed by the sigmoid cavity at the inner side of the lower 

 end of the radius receiving the head of the ulna. The articular surfaces are 

 covered by a thin layer of hyaline cartilage, and connected by a capsule, portions 

 of which are usually described as distinct ligaments. The ligaments of the 

 articulation are: 



Anterior Radioulnar. Posterior Radioulnar. 



Triangular Articular Disk. 



The anterior radioulnar ligament (Fig. 253) 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 radioulnar ligament (Fig. 254) extends between similar points 

 on the posterior surface of the articulation. 



The triangular articular disk (discus articularis} (Figs. 252 and 256) 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 periphery 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 upper 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 articu- 

 lates with the cuneiform and inner part of the semilunar bone. Both surfaces 

 are covered by a synovial membrane the upper surface by one peculiar to the 

 radioulnar articulation; the under surface, by the synovial membrane of the 

 wrist. 



