THE SKELETON 27 



fits into the glenoid cavity of the scapula. The joint thus formed 

 permits of a greater freedom of movement than any other joint in 

 the body. This is due to the fact that the head of the humerus 

 is much larger than the shallow cavity which receives it. On this 

 account, too, the shoulder-joint would be very easily dislocated, 

 were it not that the two processes of the scapula bend over the 



Fig. 28. The Bones of the Eight Fore- 

 arm in Supination. 

 Fig. 29. In Fronation. 



i, humerus ; 2, ulna ; 3, radius ; 4, carpal bones ; 

 5, metacarpal bones ; 6, phalanges. 



Fig. 30. The Bones 

 of the Fore-arm, 

 with the Orbicular 

 Ligament. 



i, ulna ; 2, radius ; 3, ole- 

 cranon process ; 4, orbi- 

 cular ligament. 



head of the humerus and prevent its getting out of joint when too 

 great pressure is brought to bear on it. 



In the forearm there are two bones the ulna and the radius. 

 The ulna (Lat. the elbow) is thick at its upper extremity, which 

 forms a hinge joint (a joint which allows of motion in one plane 

 only, like the hinge of a door) with the lower head of the humerus. 

 This extremity of the ulna also sends a projection behind the 



