MOTION. 21 



these levers with us all the time. Hence the desirability 

 of having them as light as is consistent with the requisite 

 degree of strength. The body follows the same law of 

 mechanics that we use outside of the body. A hollow 

 pillar or hollow tube has a greater strength than the same 

 amount of material in the form of a solid cylinder. The 

 long bones of the limbs are hollow, and near their ends, 

 where we have found that they need to be enlarged, we 

 find a spongy structure, where lightness and strength are 

 secured by the interlacing fibers and plates of bony 

 material. 



Uses of Bones. The part that the bones play is of a 

 passive nature ; they support the tissues, protect some 

 parts, and serve as levers on which the muscles act We 

 may not call the bones dead tissues, for they receive blood 

 and grow. But the active muscles use them as a man uses 

 a crowbar, as a mere tool. It will therefore be more 

 interesting to return to the muscles, and learn the causes 

 and conditions of their activity. 



What makes Complex Muscular Action Harmoni- 

 ous. Have you ever seen two persons, each using the 

 right hand, try to, sew, one holding the cloth, the other 

 using the needle ? Would they get along well ? Suppose 

 one were to hold the needle, and the other were to try to 

 thread it, each using one hand ? Why is it that the right 

 hands of two persons cannot work so well together as the 

 right and left hands of one person ? What connection is 

 there between the two, that one knows just what the other 

 is doing and when it does it ? Why can two individuals 

 never, with any amount of practice, work so in unity as the 

 parts of the individual ? 



