122 PHYSIOLOGY. 



The matter is well known, and the theory is extremely sim- 

 ple. The mind can apply but to one thing at once, so if it is 

 attached to a single sensation, it is brought very nearly to the 

 former state of the total absence of impressions. It is probable 

 that not only sensations in general, but that indeed a variety 

 and succession of impressions are necessary here ; if the intel- 

 lectual operations, therefore, are for the most part suspended, a 

 state near to that of the absence of impressions is induced ; or 

 if it is a sensation that has no consequence in thought or action, 

 if it is a state with some uniformity succeeding another, and 

 presents to us no reflections that lead to any train of intel. 

 lectual operations, or if it leads to no desire or aversion, there- 

 fore to no action, it will operate in the same manner by redu- 

 cing us nearly to a state of the absence of impression. The in- 

 stances are matter of common observation ; if a man endeavours 

 to attend to a discourse pronounced in a language he does not 

 understand, and with a sort of monotony, or, although the 

 language is understood, if he is not engaged in pursuing the 

 same train, it is truly to him a vox, et prceterea nihil, sounds 

 that lead to no action. Thus, what we call a dull sermon, is 

 one that does not engage the intellectual operations of the 

 hearers. Of the same kind are certain motions of the body 

 which interrupt the train of thought ; thus, the rocking of 

 a cradle interrupts every intellectual operation, while exter- 

 nal impressions are avoided. Persons carried along in an 

 easy carriage on a smooth road are very effectually induced to 

 sleep ; and if we turn a chicken's head under its wing, and give 

 it two or three undulatory motions, we have removed the exter- 

 nal impressions, and interrupted the internal operations, and 

 the chicken falls asleep. These circumstances then are mani- 

 festly such as merely diminish the motions that otherwise should 

 take place in our nervous system. 



" * The finished gratification of all vehement desires? " 

 Aristotle has observed, omne animal post coitum triste ; if the 

 coition completes the gratification, it not only makes the animal 

 ' triste J but somnolentum also. I know that different explanations 

 of this may be offered ; we may fancy that the seminal fluid is 

 of a singularly spirituous nature, and gives a particular waste 

 of the animal spirits ; but that is no ways founded, and every 



