THE HINGES. 91 



at g, to which it is tied by cords, called liga- 

 ments, particularly by one which goes. round it 

 like a band. The ends of these two bones, 

 thus united, turn on the end of the upper one, 

 which is rounded and fitted for the purpose, as 

 you may see at/. They are kept together in 

 a living person, (as indeed all bones are,) by 

 broad and short straps or cords, called liga- 

 ments, which grow to each end of the bone a 

 little way from the joint, and are very tight and 

 strong, and yet not so tight as to hinder a 

 proper motion. 



But a ball and socket joint is rather the most 

 curious. The bone which is represented at 

 b, is the scapula, or shoulder-blade. The 

 hollow place at e, is the socket in which the 

 round end or ball a, of the upper bone of the 

 arm, (the humerus,) plays freely, when we 

 move the arm. The socket is so shallow, and 

 the ligaments so long, in order to enable us to 

 make almost every kind of motion with our 

 arms, that it is much more easily slipped out of 

 joint, or dislocated, than the hinge joints are. 

 Even the hip joint, which is also a ball and 

 socket joint, has a much deeper socket ; and it 



