LETTERS TO BROTHER JOHN. 75 



stretch^ and the other only after having 1 been bent : 

 they can only contract after having been put into 

 an unnatural condition. In their natural condition 

 they are, like all other inorganic matter, at rest ; 

 and can neither contract, nor expand, nor dilate, 

 without being first submitted to the action of me- 

 chanical or chemical force. These, therefore, are 

 merely elastic. But living matter can do much 

 more than this. When at perfect rest, and in its 

 natural state, it can contract, shrink, and, in 

 short, perform spontaneous movements, merely on 

 being excited, stimulated, or irritated, without the 

 agency of any mechanical or chemical power. It 

 does thisjby virtue of a property called contractility. 

 When you look at a very strong light, the iris, the 

 coloured part of the eye, being irritated by the 

 rays of this strong light, contracts, and almost 

 closes the pupil ; that is, the black spot in the eye, 

 which is, in fact, a round hole. When your will 

 directs your arm to move, the muscles of the arm, 

 stimulated (that is, excited) by your will, contract, 

 and raise the arm accordingly. When the blood 

 rushes into the right side of your heart, that part 

 of the heart contracts, and pushes it into the left 

 side : then the left side contracts, and pushes it into 



the aorta: then the aorta contracts, and pushes it 

 E 2 



