BALL AND SOCKET JOINf. 63 



is called a ball and socket joint, and allows of a greater 

 variety of movement than any other kind. Through 

 movements taking place at it the thigh can (1) be flexed, 

 that is, bent so that the knee approaches the chest, and 

 (2) extended or straightened again ; it can (3) be abducted 

 so that the knee is moved away from the middle line of 

 the body, and (4) adducted or brought back again; by 

 movement at the hip the limb can also (5) be circum- 

 ducted, so that, with knee and ankle joints held rigid, the 

 whole leg is made to describe a cone, of which the apex is 

 at the hip-joint and the base at the foot ; and finally (6) 

 rotated so that the whole limb can be rolled to and fro a 

 little about its own long axis. All ball and sockets joints 

 allow all these movements to a greater or less extent. 



Another important ball and socket joint is that be- 

 tween the upper end of the humerus and the hollow 

 (glenoid fossa) near the upper outer corner of the shoulder 

 blade. The glenoid fossa being much shallower than the 

 acetabulum the range of movement possible at the shoulder, 

 is greater than at the hip- joint. 



Hinge-joints. In this form the bony cavities and pro- 

 jections are not spherical, but are grooved and ridged so 

 that one bone can glide over the other in one plane only, 

 to and fro, like a door on its hinges. 



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

 straightened, in technical language, flexed and extended. 



What kind of joints allow of the freest movement? What is 

 meant by flexion of the thigh? By extension? By abduction? By 

 adduction? By circumduction? By rotation? What movements 

 do all ball and socket joints permit? 



What sort of a joint is that at the shoulder? Why is more move- 

 ment possible at it than at the hip-joint? 



What is a hinge- joint? Give an illustration. 



Name other hinge-joints. 



