THE HINGES. 9 



or thigh bone moves. This round head is 

 drawn back from the bottom of the socket a 

 little way, in order to show the round ligament 

 near a. The latter is a very tough, strong 

 cord, fixed by one end at the bottom of the 

 socket, very firmly, and by the other, fastened 

 to the round head of the femur. If it were 

 not for this ligament, this joint would be dislo- 

 cated, or slipped out of its place, a thousand 

 times as often as it now is. At present, this 

 very seldom happens. I ought also to say 

 that there is a tough, gristly rirn around the 

 socket at the hip, which greatly increases its 

 depth. This socket is called the acetabuhm ; 

 meaning vinegar cup. It was supposed to 

 resemble one kind of ancient vinegar cup, in 

 use, I think, among the Romans. 



I am now going to show you a picture both 

 of a ball and socket joint and a hinge joint 

 the shoulder and the elbow. It must be 

 confessed, however, that the hinge joint at 

 the elbow is not quite so plainly seen as I 

 could wish. But you have all seen door hinges 

 no doubt, and the principle is the same. I 

 know not but the first mechanic that ever 

 8* 



