OF THE JOINTS. 219 



penter or joiner cuts his material, could enable 

 them to withstand the motions of the body, where 

 it is so utterly impossible to estimate the forces, 

 or to calculate upon the variety in the motion. 



That the varieties in the forms of the bones are 

 not irregular, nor accidental, but are related to the 

 motions to be performed, is apparent in the close 

 examination of the human skeleton, and still more 

 clearly evinced by comparative anatomy. 



The shapes of the bones are very closely re- 

 lated to the motions to be performed by the dif- 

 ferent joints. Let us observe the enlargement of 

 the diameter of the bone at an articulation. This 

 expansion of the articulated surface of the bone 

 gives power to the binding ligaments, by remov- 

 ing them from the centre of motion ; and by the 

 increase of surface and additional strength of liga- 

 ment, the danger of dislocation is much diminish- 

 ed. The friction of two bodies whose surfaces 

 move upon one another is not increased by the 

 extent of surface, the pressure remaining the 

 same. Hence the enlargement of the surfaces of 

 the joint is attended with greater security without 

 there being additional friction. But, for the most 

 part, the surfaces of the bones, instead of sliding 

 upon one another, have a rounded form, and roll 

 upon each other. Now the friction, in this case, 

 depends upon the diameter of the body which 

 rolls, and is small in proportion as the diameter 

 of that body is great, the weight being the same. 

 By this we see that the large bones forming the 



