52 



MOTION AND LOCOMOTION 



various ways, in order to lift the body and increase the 

 length of the step. In insects, the legs are tubes of 

 horny substance, within which lie the muscles for mov- 



FIG. 42. Angle worm progressing by contracting and extending body. 



ing them. In all the higher animals, the legs are stiffened 

 by bones, which make possible long steps and corre- 

 spondingly rapid progress. The bones are moved by 

 muscles surrounding and protecting them and act as 



FIG. 43. Measuring worm ready to " step" forward. 



levers greatly to increase the range of movement brought 

 about by the contraction of the muscles. 



Definition of lever. A lever is a device for the 

 application of force (power) to overcome resistance 

 (weight) by utilizing a third force or support (fulcrum). 

 Thus in every case there must be three forces acting at 

 different points on a lever. 



Law of leverage. The simplest form of lever is that 

 used by children in a see-saw. The weights of the two 

 children are the two forces acting downward upon the 

 ends of the board, and the support upon which the middle 

 of the board rests is the third force acting upward. It 



