The Muscles and the Bones. 33 



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 balance, 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 

 bent 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 force exerted 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. Our effort in moving forward is 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 comoine Lightness and Strength. The muscles, 

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

 levers with us all the time. Hence the desirability of hav 

 ing them light as well as strong. A hollow pillar or hoi- 



