20 PHYSIOLOGY. 



of the muscles act in keeping the body upright. Our 

 weight, or, we would better say, the force of gravity, is 

 continually trying to pull us down to the ground. The 

 joints are all freely movable, and hence as soon as the 

 muscles cease to act properly, in balancing against each 

 other, we lose our equilibrium, and fall if we do not 

 quickly regain it. 



Walking. In walking, we lean forward, and if we take 

 no further action we fall. But we keep one foot on the 

 ground, pushing the body forward, while the other leg is 

 flexed and carried forward to save us from the fall. We 

 catch the body on this foot, and repeat the action. To 

 show how we are really repeatedly falling and catching 

 ourselves, recall how likely one is to fall if some obstacle 

 is placed in the way of the foot as it moves forward to 

 catch the weight of the body. 



Running. In running, the action is more vigorous. 

 The propulsion by the rear leg is now greater. It gives 

 such a push as to make the body clear the ground, whereas 

 in walking, the rear foot is not lifted till the front foot 

 touches the ground. But in running there is a time when 

 both feet are off the ground. 



Locomotion by Reaction. Take two broomsticks and place them 

 crosswise under the ends of a board. Run along the board. This 

 shows that the direct effort in running is to push one's support from 

 under him. When a horse plunges forward in the mud, he only thrusts 

 his feet farther into the mud. Our effort in progression is primarily to 

 push the earth out from under us, and it is by reaction that we go 

 forward. It is the same problem with the fish swimming forward by 

 striking backward and sideways against the water, and with the bird 

 beating downward and backward upon the air. 



Bones combine Lightness and Strength. The mus- 

 cles, then, make use of the bones as levers. We carry 



