264 



THE ARTICULATIONS 



parts at right angles to each other; one between the radius and ulna, and the 

 other between the ulna and the articular disc (triangular fibro-cartilage) . 



The ligaments are: — 

 Anterior radio-ulnar. Posterior radio-ulnar. 



Articular disc. 



The articular disc (triangular fibro-cartilage) (figs. 303 and 304) assists the 

 radius in forming an arch under which is received the first row of carpal bones. 

 Its base is attached to the margin of the radius, separating the ulnar notch from 

 the articular surface for the carpus, while its apex is fixed to the fossa at the base 

 of the styloid process of the ulna. It gradually and uniformly diminishes in 

 width from base to apex, becoming rounded where it is fixed to the ulna; it is 

 joined by fibres of the ulnar collateral ligament of the wrist. 



The articular disc is about three-eighths of an inch (1 cm.) wide, and the same from base to 

 apex; thicker at the circumference than in the centre; smooth and concave above to adapt 



Fig. 299. — ^Lower Extremities op the Radius and Ulna to Show the Relation of 

 THE Articular Capsule of the Wrist Joint (in red) to the Epiphysial Lines. Note the 

 upward extension of the membrana saccrformis. 



itself to the ulna, and smooth and slightly concave below to fit over the triquetral bone. Its 

 anterior and posterior borders are united to the anterior and posterior radio-ulnar and radio- 

 carpal ligaments. It is the most important structure in the inferior radio-carpal articulation, 

 as it is a very firm bond of union between the lower ends of the bones, and serves to limit their 

 movements upon one another more than any other structure in either the upper or lower radio- 

 ulnar joints. Its structure is fibrous at the circumference, while in the centre there is a prepon- 

 derance of cells. It differs from all other fibro-cartilages in entering into two distinct articula- 

 tions; and separates entirely the synovial membrane of the radio-ulnar joint from that of the 

 wrist. 



The lower end of the interosseous membrane extends between the ulna and 

 radius immediately above their points of contact. Transverse fibres between the 

 two bones form a sort of arch above the concave articular facet of the radius, and, 

 joining the anterior and posterior radio-ulnar ligaments, complete the articular 

 capsule of the inferior radio-ulnar joint. The ligaments represent merely 

 thickenings of the capsule. 



The anterior radio-ulnar ligament (fig. .300) is attached by one end to the anterior edge 

 of the ulnar notch of tlin radius, and by the other to the rough bone above the articular surface 

 of the ulna as far medially as (lie notch, as well as into the anterior margin of the trianguar 

 cartilage from base to ajjcx. 



The posterior radio-ulnar ligament (fig. 301) is similarly attached to the posterior margin 

 of the ulnar notch at one end, and at the other to the rough bone above the articular surface of 

 the extremity of the ulna as far medially as the groove for the extensor carpi ulnaris, with the 

 sheath of which it is connected, as well as into the whole length of the posterior margin of the 

 articular disc. Both the radio-ulnar ligaments consist of thin, almost scattered, fibres. 



I 



