COVERING OF THE HOUSE. 125 



In one respect, a muscle does not shrink 

 a rope ; for the latter, when it shortens, 

 or grows larger, swells all the way alike ; but 

 when a muscle contracts to draw up a limb, 

 it swells chiefly in the middle. Some mus- 

 cles do not swell so much as this, when they 

 shorten, but they are all enlarged more or less, 

 when they move any part of our body. 



Perhaps you do not yet understand how a 

 muscle, by contracting or shortening, pulls up 

 the arm. I will endeavor to make it more 

 plain. 



I now sit at my table my right arm lying 

 on it. For the sake of explanation, I will con- 

 sider it as helpless as a stick of wood. Now 

 if I wish to get my hand to my head, how is 

 it to be done ? If a piece of dry rppe, fastened 

 by one end at the shoulder, and by the other 

 to my hand, \vere moistened, it would shrink 

 a little, and raise my hand a little way from 

 the table, but not very far. 



But suppose the lower end of the rope 



were fastened round the middle of my arm, 



and then made to shrink ; would it not raise 



the hand higher than before I mean, if the 



11* 



