120 PHYSIOLOGY. 



similar cause as that which gives natural sleep, the excitability 

 is hardly more than in cases of compression. In an evening, 

 when a person has been long in the erect posture, and the im- 

 pulse of the blood to the head has been considerably weakened, 

 yet an insuperable propensity to sleep comes on, with all the 

 symptoms of empty 'vessels of the head, and a pale and flaccid 

 countenance ; so that there is no evidence that at this time the 

 vessels of the brain are more full than upon any other occasion. 

 When we consider also how often sleep can be brought on at 

 any tune of the day by different causes, and its periodical returns 

 fixed, there can be no foundation for supposing a compression 

 either in one case or other." 



C XX VI I. As it is therefore probable that sleep and wak- 

 ing do not depend upon a different quantity of the matter of 

 the nervous fluid for the time present in the system (CXXV.), 

 or upon any causes interrupting its motion, while the condition 

 of the matter remains the same (CXXVL), we are disposed to 

 believe, that those states of sleep and waking depend upon the 

 nature of the nervous fluid itself capable of becoming more or 

 less moveable ; that it is chiefly in the brain susceptible of these 

 different conditions ; and that, especially by its condition there, 

 it has its more general effects on the whole system. 



C XX VI II. This may perhaps be confirmed by considering 

 the remote causes of sleep and waking : And it appears, that 

 cold, the absence of impressions, attention to a single sensation, 

 or to sensations that have no consequence in thought or action, 

 the finished gratification of all vehement desires, sedative sen- 

 sations and impressions, evacuations, relaxation, and any vio- 

 lent, frequent, or long continued exercise of the animal power, 

 are all of them, severally or together, causes inducing sleep. 



"Here I have enumerated all the remote causes of sleep, at 

 least all those which have been assigned, and they are so many, 

 that if they all apply, it will not be an exception to our opinion, 

 even if some are overlooked. And first, with regard to cold. 



" It is a fact ascertained in cold countries, that when cold in- 

 duces death, it always does so by first inducing sleep. Now, if 

 sleep is produced by cold, it is enough to say that cold is a power 

 operating directly upon the nervous system. None will under- 

 take to shew that cold causes death only by coagulating the 



