SECT. XVIII. 4. $. OF SLEEP. 



4. As the larger mufcles of the body are much 

 more frequently excited by volition than by fenfa- 

 tion, they are but feldom brought into action in 

 our fleep : but the ideas of the mind are by habit 

 much more frequently connected with fenfation 

 than with volition ; and hence the ceafelefs flow of 

 our ideas in dr-eams. Every one's experience will 

 teach him this truth, for we all daily exert much 

 voluntary muicular motion: but few of mankind 

 can bear the fatigue of much voluntary thinking. 



<j. A very curious circumftance attending thefe 

 our flceping imaginations is, that we feem to re- 

 ceive them by the fenfes. The mufcles, which are 

 fubfervient to the external organs of fenfe, are con- 

 nected with volition, and ceafe to aft in fleep; 

 hence the eyelids are clofed, and the tympanum of 

 the ear relaxed : and it is probable a fimilarity of 

 voluntary exertion may be neceflary for the percep- 

 tions of the other nerves of fenfe ; for it is obfeived 

 that the papillae of the tongue can be feen to be- 

 come ereded, when we attempt to tafte any thing 

 extremely grateful. Hewfon Exper. Enquir. V. 2. 

 186. Albini Annot. Acad. L. i. c. 15, Add to 

 this, that the immediate organs of fenfe have no 

 objects to excite them in the darknefs and filence 

 ot the night ; but their nerves of fenfe neverthelefs 

 continue to poflefs their perfecl activity fubferyient 

 to all their numerous fenfitive connections. This 

 vivacity of our nerves of fenfe during the time of 

 fleep is evinced by a circumftance, which almoft 

 every one mud at foine time or other have experi- 

 enced ; that is, if we fleep in the day-light, and 

 endeavour to fee fome object in our dream, the 

 light is exceedingly painful to our eyes ; and after 

 repeated druggies we lament in our fleep, that we 

 cannot fee it. In this cafe I apprehend the eye lid 

 jis in fome degree opened by the vehemence of our 

 (enfationsj and, the iris being dilated, the optic 



nerve 



