LEVERS IN THE BODY. 83 



always longer than the weight-arm, WF, and so a com- 

 paratively weak force can overcome a considerable resist- 

 ance. There is, however, a loss in rapidity and extent of 

 movement, since it is obvious that when P is raised a 

 certain distance W will be raised less. As an example of 

 this kind of lever we may take the act of standing on 

 the toes. Here the foot is the lever, and the fulcrum is 

 where its fore part rests on the ground ; the weight is 

 that of the body, and acts downwards through the ankle 

 joint at Ta, Fig. 19 ; the power is the great muscle of the 

 calf of the leg pulling by its tendon, which is fixed to the 

 end of the heel bone, Ca. 



Levers of the third order. In these (Fig. 38), the 

 power is applied between the fulcrum and the weight ; 

 hence the power-arm PF 9 is always shorter than the 



W F 



FIG. 38. A lever of the third order. F, fnlcrnm ; P, power ; W, weight. 



weight-arm, WF. The moving force acts at a mechani- 

 cal disadvantage, but swiftness and range of movement 

 are gained ; this is the form of lever most commonly used 

 in the body. For example, when the forearm is bent up 

 towards the arm the fulcrum is the elbow joint (Fig. 29) ; 



What is the mechanical gain in such levers? What is the loss? 

 Give an example of employment of a lever of the second order in 

 the body, pointing out fulcrum, point of action of the weight, and 

 point of application of the power. 



Describe a lever of the third order. What is lost and what 

 gained by it? Is it often used in the body? Give an example 



