BONES AND THEIR SOCKETS. 



19 



ends are rounded and are provided with a smooth car- 

 tilaginous covering. The smooth ends of two contiguous 

 bones fit into each other, so that when the surfaces of 

 the two ends glide the one over the. other, the two 

 bones are capable of motion 

 in opposite directions. The 

 point of attachment between 

 two bones is called the socket ; 

 and the surfaces of the two 

 ends of the bones where they 

 touch each other are called the 

 socket surfaces. The motion 

 which these bones have the 

 power of exercising in opposite 

 directions varies with the form 

 of these, socket surfaces. When 

 the surface of the socket is of 

 semi-spherical form, the motion 

 is most free, and can be exert- 

 ed backward or forw^ard in any 

 direction. The socket in this 

 ease is called a ball- or nut- 

 socket. An example of this sort 

 may be seen at the upper end 

 of the bone of the upper arm. Fig. g. The boxes of the 

 where it ends in a ball-shaped 



surface which is applied to a "■ ^^S.lLT TbIZT ,t 

 corresponding socket surface in ll%lTeT^^L:lS::^r'' ""' 

 the shoulder blade. In other 



cases motion can only take place in a definite direc- 

 tion, as, for instance, in the case of the socket con- 

 necting the U23per and fore arms. These are called 

 hinge-sockets. They serve to increase or decrease the 



