LETTERS TO BROTHER JOHN. 77 



which the several parts of the body are able to 

 move, and perform those actions which are proper 

 to them. 



A third property, distinguishing organized from 

 inorganized matter, is sensibility. 



This is exceedingly slippery ground, and ren- 

 dered still more dangerous by the darkness in 

 which it is enveloped. I shall therefore hasten off 

 the ice as quickly as possible, lest some invisible 

 straw or other should trip up my heels. 



" Irritatio," says Glisson, " est perceptio, sed 

 sensatio est perceptio perceptionis ;" that is, " Irri- 

 tation is perception, but sensation is the percep- 

 tion of a perception." Said I not it was slippery 

 ground ? But Dr. Fletcher, speaking of this defi- 

 nition of Glisson, says, " for either terseness or 

 accuracy, it cannot, perhaps, be improved." To me, 

 however, I confess, it has very much the appear- 

 ance, not of splitting a hair that is but a trifle 

 but of splitting the very ghost of a hair, which is 

 no trifle. Lobstein defines sensibility as "facul- 

 tatem stimulum percipiendi ;" that is, " the faculty 

 of perceiving a stimulus." You probably know 

 that any thing which irritates or excites any part 

 of the body to action is called a stimulus. I think 

 Lobstein is right. Thus the heart, by virtue of 



