

RADIOULNAR ARTICULATIONS 325 



Movements. The movements allowed in this articulation are limited to rotatory movements 

 of the head of the radius within the ring formed by the annular ligament and the radial notch 

 of the ulna; rotation forward being called pronation; rotation backward, supination. Supination 

 is performed by the Biceps brachii and Supinator, assisted to a slight extent by the Extensor 

 muscles of the thumb. Pronation is performed by the Pronator teres and Pronator quadrat us. 



Middle Radioulnar Union. The shafts of the radius and ulna are connected 

 by the Oblique Cord and the Interosseous Membrane. 



The Oblique Cord (chorda obliqua; oblique ligament) (Fig. 329). The oblique 

 cord is a small, flattened band, extending downward and lateralward, from the 

 lateral side of the tubercle of the ulna at the base of the coronoid process to the 

 radius a little below the radial tuberosity. Its fibers run in the opposite direction 

 to those of the interosseous membrane. It is sometimes wanting. 



The Interosseous Membrane (membrana interossea antebrachii) . The interosseous 

 membrane is a broad and thin plane of fibrous tissue descending obliquely down- 

 ward and medialward, from the interosseous crest of the radius to that of the ulna; 

 the lower part of the membrane is attached to the posterior of the two lines into 

 which the interosseous crest of the radius divides. It is deficient above, commencing 

 about 2.5 cm. beneath the tuberosity of the radius; is broader in the middle than 

 at either end ; and presents an oval aperture a little above its lower margin for the 

 passage of the volar interosseous vessels to the back of the forearm. This mem- 

 brane 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 cord 

 is a gap, through which the dorsal interosseous vessels pass. Two or three fibrous 

 bands are occasionally found on the dorsal surface of this membrane; they descend 

 obliquely from the ulna toward the radius, and have consequently a direction 

 contrary to that of the other fibers. The membrane is in relation, in front, by 

 its upper three-fourths, with the Flexor pollicis longus on the radial side, and with 

 the Flexor digitorum profundus on the ulnar, lying in the interval between which 

 are the volar interosseous vessels am] nerve; by its lower fourth with the Pronator 

 quadratus; behind, with the Supinator, Abductor pollicis longus, Extensor pollicis 

 brevis, Extensor pollicis longus, Extensor indicis proprius; and, near the wrist, 

 with the volar interosseous artery and dorsal interosseous nerve. 



Distal Radioulnar Articulation (articulatio radioulnaris distalis; inferior radio- 

 ulnar joint). This is a pivot-joint formed between the head of the ulna and the 

 ulnar notch on the lower end of the radius. The articular surfaces are connected 

 together by the following ligaments: 



The Volar Radioulnar. The Dorsal Radioulnar. 



The Articular Disk. 



The Volar Radioulnar Ligament (anterior radioulnar ligament) (Fig. 334). This 

 ligament is a narrow band of fibers extending from the anterior margin of the ulnar 

 notch of the radius to the front of the head of the ulna. 



The Dorsal Radioulnar Ligament (posterior radioulnar ligament) (Fig. 335). 

 This ligament extends between corresponding surfaces on the dorsal aspect of the 

 articulation. 



The Articular Disk (discus articularis; triangular flbrocartilage) (Fig. 336). The 

 articular disk is triangular in shape, and is placed transversely beneath the head 

 of the ulna, binding the lower ends of the ulna and radius firmly together. Its 

 periphery is thicker than its center, which is occasionally perforated. It is attached 

 by its apex to a depression between the styloid process and the head of the ulna; 

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

 the ulnar notch from the carpal articular 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 con- 





