LOCOMOTION. 27 



do not ordinarily carry handspikes and crowbars with us ; we 

 get them when we need them : but we carry 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 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 appearance, where lightness and 

 strength are secured by the interlacing fibers and plates of 

 bony material. 



As we have seen, the part that the bones play is of a pas- 

 sive nature j they support the other organs, protect some parts, 

 and serve as levers on which the muscles act. We may not 

 call the bones dead organs, 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 try to learn the causes and conditions of 

 their activity. 



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 ? In dissecting, suppose one holds a delicate object 

 with forceps while the other uses the scissors ? Or that A 

 holds the violin and does the fingering, while B manages the 

 bow ? Or in so comparatively simple a matter as the use of 

 the knife and fork in eating, how if one holds the piece of 

 meat with the fork while the other tries to cut it ? 



Why is it that the right hands of two persons cannot 

 work so well together as the right and left hands of one per- 

 son ? What connection is there between the two, that one 



