MOTION". 17 



the power is the calf muscle. We may find many exam- 

 ples of levers in the body if we look for them. 



(1) Tapping on Floor. (2) Rising on Toe. (3) Lifting Weight. 



Fig. 6. Three Kinds of Levers as shown by the Foot. 

 P Power. W Weight. F Fulcrtim. 



Kinds of Levers shown by the Foot. The different 

 classes of levers may be further illustrated by different 

 motions of the foot. In tapping the toes on the floor 

 while the heel is lifted, or in pressing down the ball of the 

 foot while running the treadle of a sewing machine, we 

 have an example of a first-class lever. In raising the 

 weight of the body on tiptoes, or as the foot is used in 

 taking each step, the foot is used as a lever of the second 

 class. When one lifts a weight with the toes, the foot is 

 used as a lever of the third class. These three classes of 

 levers are illustrated in the accompanying figures. 



Advantages and Disadvantages of Levers in the Body. The 



action of the bones of the forearm as a lever may perhaps be better 

 understood by the following considerations : If the arm consisted 

 merely of the biceps, suspended from the shoulder, it is evident that 

 its only action would be a straight pull. Suppose the biceps, thus 

 hanging alone from the shoulder, had a hook at its lower end, it could, 

 when it shortened, lift a weight just as far as it shortened, and no 



