SfRtrCTURE OP A JOINT 



245 



Movable joints. — There are four classes of movable 

 joints. To these, according to the method of joining, are 

 given the names, ball and socket, hinge, gliding and pivot 

 joints. The best examples of the first type are the joints 

 of the arms and legs with the shoulder girdle and 

 pelvic girdle respectively. The knee and elbow illustrate 

 the second type. The third type is found in the joinings 

 of the bones Avhich make up the WTists and ankles (carpals 

 and tarsals). These bones slide over one another and 

 thiis give flexibility to these 

 parts. A fourth type is illus- 

 trated by the union of the axis 

 and atlas and the ulna and 

 radius at the elbow. 



Structure of a joint. — The 

 joint in a leg of lamb fur- 

 nishes a very good example 

 of the structure of a joint. 

 After all the flesh is removed 

 the two bones will be seen 

 to be held together by tough 

 bands of connective tissue 

 called ligaments. These bands 

 are so arranged as to permit 

 of motion in certain direc- 

 tions, and at the same time 

 prevent too great a separation of the bone ends. Cut- 

 ting away these bands, the smooth ends of the bones 

 with their caps of cartilage are seen. These caps prevent 

 friction where the two surfaces rub together. As the 

 ligaments are cut there escapes a thin, slimy liquid of the 

 consistency of a white of egg. This liquid is called the 



Fig. 99 — Section of a hinge joint. 



