128 LECTURE III. 



bend the elbow, we generally wish to ap- 

 proximate or apply the hand to some part 

 of our persons ; and in consequence of the 

 oblique direction of the pulley, we accom- 

 plish by one motion, what would have 

 required two, had the groove been formed 

 straight, as it is in animals. That part of 

 the ulna which is next the radius, for so the 

 other bone of the fore-arm is called,- is hol- 

 lowed out into a semicircular cavity, in 

 which the radius revolves to a certain 

 degree. The top of the radius is formed 

 into a shallow cup, which fits on to a 

 convexity of much greater extent of 

 surface in the bone of the arm. There- 

 fore, the cup can move forwards and back- 

 wards, in correspondence with the motions 

 of the ulna, and can also, in any state of 

 flexion or extension of the joint, turn round 

 upon its own axis. The radius has also a 

 semicircular convex surface, which is lodged 

 in the excavation of the ulna, and at the 

 same time revolves in that cavity to a cer- 

 tain degree. Such is the mechanism by 

 which we are enabled in any state of flexion 

 and extension of the elbow, to turn the 



