402 MOTIONS NECESSARY IN WALKING. 



Notwithstanding these facts, the action of the gastrocnemius 

 and soleus is essential whenever we raise the heel from the 

 ground, while the weight of the body presses on the front part 

 of the foot ; and it then acts with a force which equals, if it 

 does not exceed, the weight of the body. 



Jumping, at the first view of it, appears an extraordinary 

 operation ; but if a man who lies on the ground, with his feet 

 against a wall, makes a muscular exertion, such as is necessary 

 for jumping, the nature of the operation is very intelligible. It 

 is a sudden extension of the feet and knees, and sometimes of 

 the trunk of the body. The stroke is made against the wall ; 

 but as that does not yield, the whole motion is impressed upon 

 the body ; which is projected from the wall horizontally in the 

 same way that in jumping, it is projected from the ground 

 vertically. 



