BALL-AND-SOCKET f OIN TS. HINGE-JOINTS. 37 



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femur rolls very easily in the hollow, or socket. The carti- 

 lage forms a yielding cushion which hinders the bones 

 from scratching or chipping one another. 



To keep the bones in place and prevent too free move- 

 ment, strong bands of connective tissue, called ligaments, 

 unite them, being fixed above to the hip-bone and below 

 to the femur. Many powerful muscles also pass from one 

 bone to the other, and keep them pressed close together. 



4. Ball-and-Socket Joints. A joint like that at the 

 hip, where the round end of one bone fits into a cavity 

 in which it can roll in any direction, is called a ball-and- 

 socket joint. It allows more free movement than any 

 other kind. At the shoulder there is another ball-and- 

 socket joint. 



5. Hinge-Joints In hinge-joints the ends of the bones 

 are not evenly rounded on all sides, but one bone has 

 projecting ridges which slide in grooves on the other. 

 The result is that the only movements possible are to and 

 fro, or in one direction and back again, like a door on its 

 hinges. 



The knee is a hinge-joint: it can only be bent and 

 straightened; or, as physiologists say, flexed &\\& extended. 

 Between the phalanges of the fingers there are other 

 hinge-joints. 



6. Pivot-Joints. In pivot-joints one bone rolls round 

 another. 



A good example is the joint which permits us to turn 

 the head from side to side. 



The uppermost vertebra (Fig. 14), which carries the 



4. What is a ball-and-socket joint ? 



5. Describe a hinge-joint. Examples. 



6. What are pivot-joints ? Describe the atlas. What is the odon- 



