78 LETTERS TO BROTHER JOHN. 



its contractility, has the power of contracting ; but 

 it is by virtue of its sensibility that it perceives 

 the proper moment for exerting this power the 

 precise when to contract; viz. when the blood 

 stimulates it by its presence, as it rushes into its 

 cavities. Sensibility, therefore, is that property of 

 organized matter by which it becomes aware of an 

 impressing cause by which it perceives when it is 

 acted upon by a stimulus. 



In vain would the heart be organized in vain 

 would it be endowed with contractility (that is, the 

 power of acting) were it not also endowed with 

 sensibility ; that is, the power of knowing when to 

 act of feeling the presence of a stimulus. The 

 several stimuli may be likened to a number of 

 messengers sent out from " head-quarters," the 

 heart, in order to tell the several parts of the body 

 when to act ; and the arteries are the roads along 

 which they travel the principal stimuli within the 

 body being the countless streams of blood continually 

 flowing through its arteries. You must remember, 

 however, that this office of stimulation is by no 

 means the main duty which the blood has to per- 

 form : it is only an adventitious office only one 

 of the numerous functions which the blood per- 

 orms. Besides the blood, there is another remark- 



