NATURAL THEOLOGY. 91 



other prevails as may be best adapted to the motion 

 which is wanted. Thus, a hinge joint admits only of 

 a backward and forward motion. We have this joint 

 in our fingers, enabling us to open and close them 

 which is all the motion that would be of any use. It 

 would have been of no conceivable advantage to have 

 turned the joints of the fingers completely round, so 

 as to bring the nails upon the inside. At the should- 

 er, on the contrary, we want a joint which will per- 

 mit us to stretch and expand the arm in every direc- 

 tion : there we have a ball and socket joint ; where 

 the round head of the bone is received into a cup, 

 which gives it a free play on every side. At the 

 neck, a joint is necessary that will suffer the head 

 to revolve, but the spinal marrow requires that the 

 joint should -be subject to no slip, and that no loose 

 motion should be allowed it, which, in the common 

 ball and socket joint, it is difficult to avoid. Here 

 we have a pivot joint, a joint with a firm pin run- 

 ning up and accurately fitted into a cavity which al- 

 lows the motion demanded, but permits no irregular- 

 ity. Sometimes the same bone is constructed at the 

 opposite ends for different species of joints where dif- 

 ferent kinds of motions happen to be necessary. 

 Thus, the extremity of the thigh bone is made for a 

 ball and socket joint, where it is united at the hip, but 

 for a hinge joint, where it is united at the knee. The 

 utility is obvious. A hinge joint at the upper end 

 would have permitted a backward and forward mo- 

 tion, but no other; and not a step could have been 

 taken beyond a certain width, except by spreading 



