PHYSIOLOGY. 91 



would be necessary, the increase of half a grain had such effects. 

 We have many instances of the same k ; nd with regard to pur- 

 gatives, of persons at first requiring a certain dose, but who in 

 consequence of repetition are moved by a smaller dose. This 

 effect will not be disputed ; but it gives difficulty when con- 

 trasted with a law of sensation, by which we become more in- 

 sensible in consequence of the same stimulus being frequently 

 applied ; and accordingly in the very case of the stomach, a 

 circumstance arises from the frequency of the repetition, which 

 I cannot determine. In a person who is just now, by a small 

 dose, excited to vomiting, if that is repeated the stomach 

 sometimes becomes more insensible, and the dose must be in- 

 creased ; and this event, perhaps, is more frequent than the 

 other ; but they are both equally true, and perhaps no rule can 

 be made to determine the application, as it depends upon a tem- 

 perament, and perhaps an idiosyncrasy. But a curious ques- 

 tion here arises: Do stimulants, which produce facility of 

 motion, and operate more upon motion than upon sense and 

 will, act in diminished doses in consequence of repetition? 

 And do the sedatives, on the contrary, which are considered as 

 operating upon sense, require to be increased upon repetition ? 

 Thus we have an hundred instances of persons in the habit of 

 taking opiates ; and I never knew an instance of a person that 

 could be made to sleep by a smaller dose than that he had 

 taken before. But that is a question for your speculation. 



66 The doctrine we are speaking of has been as much taken 

 notice of in morals as in medicine ; and it is a maxim, that cus- 

 tom increases our active, and diminishes our passive habits." 



CXV. Are not the contractions of muscles produced by the 

 action of the animal power, those which are more especially 

 liable to become uneasy and weak by frequent repetition ? 

 " It might be naturally supposed that the lassitude or debility, 

 which is the consequence of frequently excited and long continu- 

 ed action, may depend upon some circumstance in the nature 

 of muscular fibres themselves. Till we know more of the 

 nature of their contraction we cannot say much with regard to 

 this ; for in most of the instances the animal power is concern- 

 ed, as in the case of the organs of voluntary motion. I will 

 give you a fact, which is curious as affecting this question ; 



