LEVERAGE OP BONES. 461 



each other. Thus, let r and o (fig. 214) be two bones con- 

 nected by a joint ; and let the muscle m, which moves the 

 lower bone upon the upper, be attached to the former at i. 

 Now as this muscle acts almost precisely in the line of the 

 bones themselves, almost all its 

 power will be expended in draw- 

 ing the lower bone against the 

 upper. But by the enlargement 

 of the ends of the bones, as seen 1 

 in fig. 215, the direction of the 

 tendon of the muscle m is so 

 changed, near its insertion i, that 



the contraction of the muscle will cause the lower bone to 

 turn upon the upper one with comparatively little loss of 

 power. In the knee we find a still greater change of direction 

 effected, by the interposition of a movable bone, the patella or 

 knee-pan, in the substance of the tendon. 



612. But the advantage or disadvantage with which the 

 muscles act upon the bones, depends in great degree upon the 

 relative distances of their point of attachment from the fulcrum 

 on which the bone moves, and from the point at which the 

 resistance is applied. Every bone acted-on by muscles may 

 be regarded as a lever, having its fulcrum or point of support 

 in the joint, its power where the muscle is attached to it, and 

 its weight where the resistance is to be overcome ; and the 

 distances of the fulcrum from the power and the weight 

 respectively are termed the two arms of the lever. Now, on 

 the mechanical principles fully explained elsewhere (MEGHAN. 

 PHILOS., 287), the relative length of 

 these two arms determines the force y 



which is necessary to overcome a I ... 1M ...... 



given resistance. Thus in the Steel- j J""" ^^ P 



yard (fig. 216), the beam is divided T 

 into two arms of unequal length O 

 at the point of support or fulcrum a ; 



at the end of the short arm r, hangs the body whose down- 

 ward pressure we wish to determine ; and on the other p 

 there slides a weight, which will balance a greater or less 

 amount of pressure at the opposite extremity r, according as 

 it is made to hang from a point which is more distant from 

 the fulcrum or nearer to it, that is, according as the length 



