LEVERS IN THE BODY 289 



Expressed in terms of distance from the pivot this relation 

 is known as the laiv of levei^s. 



Weight times the perpendicular distance of its point of 

 application from the pivot equals the power required to 

 balance it times the perpendicular distance of its point of 

 application from the pivot. 



Thus, in a lever of the first class a pull of one pound ap- 

 plied two feet from the pivot will exactly balance a weight 

 of two pounds applied one foot from the pivot. Further, 

 in moving the two-pound weight through a circular arc one 

 foot in length, the one-pound power moves through a two- 

 foot arc, and the actual amount of energy exerted by the 

 power and weight is the same. (See Ex. LVI.) 



On this account levers are very useful in mechanical 

 operations where they enable a person to move a large 

 weight a small distance by applying a small power through 

 a longer distance. The actual amount of energy exerted 

 at each end of the lever remains the same. 



Levers in the body. — When we come to examine the 

 method of application of the muscles to the jointed bones 

 of our body we find that nature has made use of this law 

 of levers in a very remarkable way. For example, in the 

 forearm we have a lever of the third class. (See Fig. 115, A. ) 

 Here the bones form the rigid bar, the elbow is the pivot, 

 and the power is applied by the biceps muscle attached 

 between the hand and elbow. With this arrangement the 

 muscle is able by a short contraction to raise a weight in 

 the hand no little distance. In this case the amount of 

 force exerted by the muscle to raise a given weight can 

 easily be calculated by applying the law of levers. 



Furthermore, if we put in the hand a weight which 

 the muscle is just able to raise, the calculation enables 



BDDY. PHYS. — 19 



