Chap. 10 MOVEMENT MUSCLES 167 



flexed and thrust forward to catch the falling body. At the same time the calf 

 muscle of the right leg contracts and lifts the heel. The left foot is being placed 

 on the ground and for an instant both feet are on the ground. The weight of 

 the body is now shifted to the left leg and the right one is swung forward into 



/4 



Fig. 10.10. The same pattern of movement of the arms and legs of a man, and the 

 legs of a cat, an ancient inescapable habit. 



a new position in front of the left. When the right foot is planted, the weight 

 of the body is shifted to that leg, and the pull of the muscles now lifts the left 

 heel. The left leg is then swung forward again in front of the right one. Thus 

 walking is like the movements of a pendulum repeated several thousand times 

 per day. 



Infants begin their travels on all fours as quadrupeds continue to do through 

 life. Brisk walkers swing their arms and when they do so the right arm and the 

 left leg go forward at the same time in exactly the same pattern as in a walking 

 cat and with the same muscles operating (Fig. 10.10). We cannot walk in any 

 other way; neither can the cat. Inherited pattern of the movement of muscle is 

 as inescapable as the inheritance of its structure. 



