98 MECHANISM OF THE BONES. 



THE UPPER ARM. 



The upper arm displays a different kind of mechan- 

 ism. It makes a hinge at the elbow, permitting the 



Fig. 12. 



a 



Fig. 13. 



Figure 12. 



a, the humerus, or bone of the upper arm, exhibiting the ball or 

 rounded head at the top, which is received into a socket at 

 the shoulder, forming an universal joint. JR, the radius, or 

 bone which turns with the hand, and is hinged at the wrist. 

 u, the ulna, which is hinged at the elbow, and by which we 

 perform all the motions of bending or extending the arm. 



Figure 13. 



A small portion of the humerus, and also of the radius and ulna, 

 exhibiting the structure of the elbow joint. R, being the ra- 

 dius which is loose from the joint, for the purpose of enabling 

 the hand, which is hinged at the opposite extremity, to re- 

 volve. 



arm to bend : but at the shoulder we have what is 

 called a ball and socket joint ; that is, a universal 

 joint, which will turn in every direction. If we 



