232 THE ARTICTLATIOXS 



(r) THK IXFEHIOR IvADIO-lI.NAR JoiNT 



Class. — DIttrthrosis. Subdivision. — Trochoides. 



This is, ill one ivspcet. tlie reverse of the superior; for the radius, instead of 

 presenting a eircuhir head to rotate upon the faeet on the ulna, presents a concave 

 facet which rolls round the ulna. The articulation may he said to consist of two 

 })arts at right angles to each other: one l)etween the radius and ulna, and the 

 other between the ulna and triangular fibro-cartilage. 



The ligaments are: — 



Anterior radio-ulnar. Posterior radio-ulnar. 



Triangular fi})ro-cartilage. 



The triangular fibro- cartilage (tigs. 234 and 235) assists the radius in forming 

 an arch under whit-h is received the first row of carpal bones. Its base is attached 

 to the margin of the radius, separating the sigmoid cavity from the articular 

 surfaee 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 Avhere it is fixed to the ulna; it is joined by fibres of the 

 internal lateral ligament of the wrist. It 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 itself to the ulna, and smooth and 

 slightly concave lielow to fit over the cuneiform bone. Its anterior and posterior 

 1 »( )rders are united to the anterior and posterior radio-ulnar and radio-carpal liga- 

 ments. 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 

 tlu' ujiper or lower radio-ulnar joints. Its structure is fibrous nt the circumference, 

 while in the centre there is a jDreponderance of cells. It differs from all other fibro- 

 cartilages in entering into two distinct articulations; and separates entirely the 

 synovial membrane of the radio-ulnar joint from that of the wrist. 



The anterior radio-ulnar ligament (fig. 231) is attached by one end to the 

 anterior edge of the sigmoid cavity of the radius, and ]>y the other to the rough 

 l)one above the articular surface of the ulna as far inwards as the notch, as well as 

 into the anterior margin of the triangular cartilage from base to apex. 



The posterior radio-ulnar ligament (fig. 232) is similarly attached to the 

 posterior margin of the sigmoid cavity at one end, and at the other to the rough 

 bone above the articular surface of the extremity of the ulna as far inwards as the 

 groove for the extemor carpi ulnarls, -with the sheath of which it is connected, as 

 well as into the whole length of the posterior margin of the triangular fibro-cartilage. 

 Both the radio-ulnar ligaments consist of thin, almost scattered, fibres, but they 

 serve to form a capsule for the support and protection of the synovial membrane. 



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

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

 t\vo bones form a sort of arcli above the concave articular facet of the radius, and, 

 joining the anterior and ]iosterior radio-ulnar ligaments, complete the capsule of the 

 inferior r.-uho-ulnar joint. 



The synovial membrane, sometimes called the membrana sacciformis, is 

 large and loose ni i)roportion to the size of the joint. It is not only interi)osed 

 lietween the ra(nal and ulnar articular surfaces, but lines the terminal articular 

 surface of the ulna and the upper surface of the triangular fibro-cartilage. 



The arterial supply is derived from the anterior interosseous artery, and 

 branches of the anterior carpal arch. 



The nerve-supply comes from the anterior interosseous of the median, and tlie 

 l)osteiior interosseous l)ranch of the musculo-spiral. 



The movements of the radius. — The upper end of the radius rotates upon 

 an axis drawn through its own head and neck within the collar formed bv the 

 lesser sigmoid cavity and the orl)icular ligament, while the lower end, retained in 

 position l)y the triangular ligament, rolls round the bead of the ulna. This rotation 



