LEVERAGE OP BONES. 
461 
Fig. 214. 
Fig. 215. 
each other. Thus, let r and o (tig. 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. 
hTow 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 
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. !STow, on 
the mechanical principles fully explained elsewhere (Mechan. 
Philos., § 287), the relative length of a 
these two arms determines the force 
which is necessary to overcome a 
given resistance. Thus in the Steel¬ 
yard (fig. 216), the beam is divided 
into two arms of unequal length 
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 
= 3 , 
Fig. 216. 
