BOXES OF THE FOEE-ABM. 



145 



surface of the humerus ; which acting as a stop, renders exten- 

 sion beyond the straight line impossible. The hand is attached 

 to the lower end of the radius ; and as that part was Rg 153. 

 designed to perform pronation and supination, a 

 peculiar mechanical provision was necessary for 

 these important motions. The round head of the ra- 

 dius (fig. 153, n) is bound by a firm annular liga- 

 ment to the ulna (i), and the concavity on its sur- 

 face is received in a corresponding convexity on the 

 outer condyle of the humerus. Hence both bones 

 move upon the humerus, in acts of flexion and 

 extension, whilst the radius rolls upon the ulna, 

 carrying with it the hand in pronation and 

 supination, separate sets of muscles being as- 

 signed to each class of movements. It is only among 

 the higher mammals that any motion is permitted 

 between the bones of the fore-arm. These motions = ' 

 are most important in man ; for without them 

 the hand would be incapable of a vast variety 

 of movements so necessary to the full develop- 

 ment of the purposes for which that instrument 

 was designed. When the free motions between 

 the bones of the fore-arm are impaired by injury 

 Fig. 154. 



