.&ICT. XVIII. 19. OF SLEEP. 167 



king reveries we fometimes in a moment lofe the train of thought^ 

 but continue to feel the glow of pleafure, or the deprefiion of 

 fpirits, it occafioned : whilft at other times we can retrace with, 

 cafe thefe hiftories of our reveries and dreams. 



The above explanation of furprife throws light upon this fub- 

 jeft. When we are fuddenly awaked by any violent flimulus, 

 the furprife totally difunites the trains of our lleeping ideas 

 from thofe of our waking ones ; but if we gradually awake, 

 this does not happen ; and we readily unravel the preceding 

 trains of imagination. 



19. There are various degrees of furprife ; the more intent 

 we are upon the train of ideas, which we are employed about, 

 the more violent muft be the ftimulus that interrupts them, and 

 the greater is the degree of furprife. I have objferved dogs, who 

 have flept by the fire, and by their obfcure barking and ftrug- 

 gling have appeared very intent on their prey, that (hewed great 

 furprife for a few feconds after their awaking by looking eagerly 

 around them ; which they did not do at other times of waking. 

 And an intelligent friend of mine has remarked, that his lady, 

 who frequently fpeaks much and articulately in her fleep, could 

 never recollecl her dreams in the morning, when this happened 

 to her : but that when (he did not fpeak in her fleep, (he could 

 always recoiled! them. 



Hence, when our fenfations at fo (Irongly in fleep as to in* 

 fluence the larger mufcles, as in thofe, who talk or ftruggle in 

 their dreams ; or in thofe, who are affe&ed with complete rev- 

 erie (as defcribed in the next Section), great furprife is produ- 

 ced, when they awake ; and thefe as well as thofe, who are 

 completely drunk or delirious, totally forget afterwards their 

 imaginations at thofe times. 



20. As the immediate caufe of fleep confifts in the fufpen- 

 fion of volition, it follows, that whatever diminifhes the general 

 quantity of fenforial power, or derives it from the faculty of 

 volition, will conftitute a remote caufe of fleep ; fuch as fatigue 

 from mufcular or mental exertion, which diminifhes the general 

 quantity of fenforial power; or an increafe of the fenfitive mo- 

 tions, as by attending to foft mufic, which diverts the fenforial 

 power from the faculty of volition ; or laftly, by increafe of the 

 irritative motions, as by wine, or food, or warmth ; which not 

 only by their expenditure of fenforial power diminim the quan- 

 tity of volition ; but alfo by their producing pleafurable fenfa- 

 tions (which occafions other mufcular or fenfual motions in con- 

 fequence), douoiy decieafe the voluntary power, and thus more 

 forcibly produce fleep. See Seel:. XXXIV. i. 4. 



Another method of inducing fleep is delivered in a very inge- 

 nious* 



