THE BONES AS LEVERS 



33 



Fig. 20. — Lever of the First Order 



THE BONES AS LEVERS 



The muscles concerned in locomotion are each, with few exceptions, 

 attached to two bones, either directly, or through the medium of tendons. 

 The bones connected by their extremities form a series of free-moving 

 joints, and being thus enabled to move 

 one upon the other, constitute so many 

 bony levers by which the movements 

 of the body are effected. 



A lever is a rigid and inextensible 

 l)ar used for the purpose of moving 

 bodies l)y means of a power or weight 

 either greater or less than that of tlie 

 bodies themselves. 



When a bone is operating as a 

 lever, one end is more or less fixed 

 and made to moA'e upon tlie corre- 

 sponding surface of the l)one with 

 which it is articulated, and thus per- 

 forms the office of fulcrum, while the other portion is left free to move in 

 obedience to the power and the weight. A lever presents for considera- 

 tion three elements, viz. the Fulcrum, the Power, and the AVeight, the 

 respective positions of which 

 <letermine the class to which it 

 belongs. 



The action of a lever in the 

 development of force and speed 

 will depend upon tlie relation of 

 the power to the weight. 



Levers are divided into three 

 classes. In levers of the first 

 class the fulcrum is placed be- 

 tween the power and the weight, 

 as illustrated in the beam of an 

 ordinary pair of scales (fig. 20). 



In levers of the second class 

 the fulcrum is situated at one 

 end, and the weight occupies a position between it and the power. This 

 kind is illustrated in the loaded wheel-barrow, when the man supporting 

 the shafts represents the power, the barrow and its contents the weight, 

 and the wheel the fulcrum (fig. 21). 



Fig. 21. — Lever of the Second Order 



