WALKING AND RUNNING. 



51 



ing, a lever of the first order is illustrated ; tlie hand being 

 the weight, the extensor of the elbow the power, and that 

 joint the fulcrum placed between the weight and power. 

 But when the elbow is straightened in raising the body on 

 the hands, then the superincumbent weight falls at the 

 elbow, between the extensor muscle, which is still the power, 

 and the hand, which is now the fulcrum; and the second order 

 of levers is illustrated. When a weight held in the hand is 

 raised by bending the elbow, the flexor muscles in front of 

 the joint are those which act; and, as they are situated 

 between the fulcrum and weight, a lever of the third order 

 is brought into action. 



Fig. 28. THE THREE ORDERS OF LEVERS, illustrated at the Elbow. 

 The muscles of the calf (the gastrocnemius and soleus), 

 passing down to the tenclo Achillis, are concerned in 

 actions illustrating all three kinds of lever. "When the 

 foot is raised and the toes depressed, as in working a 

 pedal, the weight is at the toes, and the ankle-joint is the 

 fulcrum of a lever of the first order ; when we rise on tip- 

 toe it is the muscles of the calf which raise the heel, the 

 fulcrum is at the toes, and the weight of the body falls on 

 the ankle after the fashion of a lever of the second order; 

 and, lastly, in the slighter action of the same muscles, when 

 the heel is kept to the ground by the weight of the body, the 

 force which prevents the body falling forwards is applied 

 at the upper attachments, while the ankle is the fulcrum, 



