1^0 Boner of tht Arm. 



of the radius with the os humeri, are very inconfider- 

 able ; the fmailer end of the one bone being oppofed 

 to the larger of the other, and depending on it for 

 fupport and firmnefs, The ends of thefe two bones 

 are clofely joined together ; their middle parts recede' 

 from each other, with the interpofition of an interofie- 

 ous ligament, fimilar to that Between the tibia and 

 fibula. At the upper end of the radius is a fmall ca- 

 vity, which receives the outer protuberance of the os 

 humerij and the projecting ridge tiirrounding this cavity 

 rolls in a fmall fmus at the upper end of the ulna, in 

 which fituation it is held by a ring of cartilage. At 

 the bottom of the radius there is alfo a fimilar finus, 

 which receives the lower end of the ulna. The radius 

 is therefore joined to the ulna by a double articulation ; 

 for, above, a tubercle of the radius plays in a focket of' 

 the ulna, whflft below, the radius affords the focket, and, 

 the ulna the tubercle. The motion, however, per- 

 formed in thefe two is very different ; for, at the upper 

 end, the radius does no more than turn round its axis> 

 while, at the lower end, it moves in a fort of cycloi.d 

 upon the round part of the ulna; and as the hand is 

 here articulated and firmly connected to the radius* 

 they muft move together. The ulna, being connected 

 by a hinge -joint to the os brachii, has fcarcely any 

 other motion than that of flexion and extenfion, in 

 which it carries with it the radius. The motions of the 

 hand, in which the palm is turned either upwards or 

 downwards, are performed by thofe of the radius on 

 the ulna, carrying with it the hand, From thefe cir- 

 cumltances it appears, that the ulna more particularly 

 belongs to the os humerij and the radius to the car- 

 pus. The ulna fometimes carries with it the radius, 

 but the radius never moves the ulna, which, like the 

 tibia is connected by a hi^ge -joint, and has motion only 



in 



