120 THE HUMAN BODY 



Levers of the Second Order. As an example of the employ- 

 ment of such levers (weight between power and fulcrum, Fig. 50) 

 in the Body, we may take the act of standing on the toes. Here 

 the foot represents the lever, the fulcrum is at the contact of its 

 fore part with the ground; the weight is that of the Body acting 



FIG. 50. A lever of the second order. F, fulcrum; P, power; W, weight. The 

 arrows indicate the direction in which the forces act. 



down through the ankle-joints at Ta, Fig. 35; and the power is the 

 great muscle of the calf acting by its tendon inserted into the heel- 

 bone (Ca, Fig. 35). Another example is afforded by holding up the 

 thigh when one foot is kept raised from the ground, as in hopping 

 on the other. Here the fulcrum is at the hip-joint, the power is 

 applied at the knee-cap by a great muscle (quadriceps femoris) 

 which is inserted there and arises from the pelvis; and the weight is 

 that of the whole lower limb acting at its center of gravity, which 

 lies somewhere in the thigh between the hip and knee-joints, 

 that is, between the fulcrum and the point of application of the 

 power. 



Levers of the Thkd Order. These are the levers most com- 

 monly used on the Human Body (power between weight and 

 fulcrum, Fig. 51). For example, when the arm is bent at the 

 elbow the fulcrum is the elbow-joint, the power is applied at 

 the insertion of the biceps muscle (Fig. 39) into the radius and of 



W 



FIG. 51. A lever of the third order. F, fulcrum; P, power; W, weight. 



another muscle (not represented in the figure, the brachialis 

 anticus,) into the ulna, and the weight is that of the forearm and 

 hand, with whatever may be contained in the latter, acting at 

 the center of gravity of the whole somewhere on the distal side 



