328 ELEMENTARY PHYSIOLOGY less. 



13. The Mechanics of Locomotion. — We may now 



pass from the consideration of the mechanism of mere 

 motion to that of locomotion. 



When a man who is standing erect on both feet pro- 

 ceeds to tvalk, beginning with the right leg, the body is 

 inclined, so as to throw the centre of gravity forward ; 

 and, the right foot being raised, the right leg is advanced 

 for the length of a step, and the foot is put down again. 

 In the meanwhile, the left heel is raised, but the toes of 

 the left foot have not left the ground when the right foot 

 has reached it, so that there is no moment at which both 

 feet are off the ground. For an instant, the legs form 

 two sides of an equilateral triangle, and the centre of the 

 body is consequently lower than it was when the legs 

 were parallel and close together. 



The left foot, however, has not been merely dragged 

 away from its first position, but the muscles of the calf, 

 having come into play, act Upon the foot as a lever of the 

 second order, and thrust the body, the weight of which 

 rests largely on the left astragalus, upwards, forwards, and 

 to the right side. The momentum thus communicated to 

 the body causes it, with the whole right leg, to describe an 

 arc over the right astragalus, on wliich that leg rests 

 below. The centre of the body consequently rises to its 

 former height as the right leg becomes vertical, and 

 descends again as the right leg, in its turn, inclines 

 forward. 



^Vhen the left foot has left the ground, the body is 

 supported on the right leg, and is well in advance of the 

 left foot ; so that, without any further muscular exertion, 

 the left foot swings forward like a pendulum, and is carried 

 by its own momentum beyond the right foot, to the 

 position in which it completes the second step. 



When the intervals of the steps are so timed that each 

 swinging leg comes forward into position for a new step 

 without any exertion on the part of the walker, walking 

 is effected with the greatest possible economy of force. 



