26 MOTION LOCOMOTION. 



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. 



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. 



In similar manner we may explain the action in jumping, 

 hopping, etc. We should distinguish between motion and 

 locomotion. In the latter we move the body from place to 

 place. It is interesting to note that our locomotion is pro- 

 duced by reaction. The direct result of pushing downward 

 and backward with one leg is simply to extend the leg. Or, 

 if there is anything in the way, to push that something out 

 of the way. If one walks on a barrel the barrel is rolled 

 backward. To show this better, take two broomsticks, and 

 lay them parallel a few feet apart ; lay a long board across 

 them ; then try to run along the board. Of course one is 

 likely to fall, and sees that the direct effect of the effort is 

 simply to push the board out from under one's self. When 

 a horse tries to spring forward in the mud, he only plunges 

 his feet deeper in the mud. Our effort in progression is pri- 

 marily an attempt to push the earth out from under us, and 

 it is only by reaction that we go forward. It is the same 

 problem with the fish swimming forward by striking back- 

 ward and sideways against the water, and in the case of the 

 bird, beating downward and backward in the air. In each 

 case progression comes only through the mechanical principle 

 of reaction. 



The muscles, then, make use of the bones as levers We 



