THE MUSCULAR SYSTEM 2$ I 



contraction. To bring about the required movements of 

 the body, which in some instances amount to four or five 

 feet, requires that a large proportion of the muscular force 

 be exchanged for motion. The machines of the skeleton, 

 while providing for motion in definite directions, also pro- 

 vide the means whereby strong forces, acting through 'short 

 distances, are made to produce movements of less force, 

 through long distances. The mechanical device employed 

 for this purpose is known as 



The Lever. The lever may be described as a stiff bar 

 which turns about a fixed point of support, called the ful- 

 crum. The force applied to the 

 bar to make it turn is called the / / a 



power, and that which is lifted ,4 " 



or moved is termed the weight. ,b 



The weight, the power, and the plj F / 



fulcrum may occupy different 

 positions along the bar and this , ,, _ 



F__ o jim 



gives rise to the three kinds of .A-^'IlT.- ^, 



levers, known as levers of the 



first class, the second class, and ^ a 



the third class (Fig. 115). In ^ ^ -"" m 



levers of the first class the f ul- 



. . FIG. 115. Classes of levers, 



crum occupies a position some- L Two leyers of first c]ass 



where between the power and showing fulcrums in different 

 the weight. In the second class positions. II. Lever of see- 

 the weight is between the ful- ond class - IIL Lever of third 



class. F. Fulcrum. P. Power. 



crum and the power. In the w Weight a ^ Power . arm . 

 third class the power is between & Weight-arm, 

 the fulcrum and the weight. 



Application to the Body. In the body the bones serve 

 as levers; the turning points, or fulcrums, are found at the 

 joints; the muscles supply the power; and parts of the 



ii 



