12(j PHYSIOLOGY. 



physiologists have not neglected the causes of waking, they 

 have been at much more pains to enumerate the causes of sleep. 

 This seems paradoxical, because sleep would appear to be the 

 natural state of the human body, and waking the unnatural 

 one, for I say, if I can keep off external impressions, there will 

 be no action, but a state of death; whereas waking always 

 depends upon external powers putting the system in mo- 

 tion, and producing, indeed, motions which contribute to sup- 

 port the same activity, but are all derived from some external 

 influence. 



" And first, ' a certain degree of heat? This is in opposition 

 to the first cause of sleep which I mentioned viz. cold. I have 

 explained the fact, that a certain degree of heat is necessary to 

 animal life, and to the waking state : the limits of which, how- 

 ever, and the degree necessary, we cannot tell. We know that 

 a certain degree of heat is an occasion of our waking, and pre- 

 vents sleep ; and that we procure sleep by cooling the body to 

 a certain degree. But the heat going further disposes to sleep 

 again ; and going still further it induces waking. In order 

 to discuss the last fact, I say that every strong impression is a 

 cause of waking ; but that heat does not by its stimulus act upon 

 the nervous system, but upon the circulating system, increasing 

 the impetus of blood in the vessels of the brain, which is one 

 of the most powerful means of waking. When the heat does 

 not amount to such a stimulus, it certainly gives a propensity 

 to sleep : and I am not positive how this is to be explained. 

 The external heat, with the concurrence of the generating power 

 within the body, commonly supports a temperature of our 

 bodies between 96 and 100. The generating power is in dif- 

 ferent circumstances, according to the external temperature. 

 Whenever this is below 62, the generating power is above its 

 ordinary degree, in order to balance ; but whenever the exter- 

 nal temperature is above 64, the generating power is constant- 

 ly diminishing, so that heat evidently is a means of inducing 

 debility or a weaker motion in the generating power of heat, 

 which, as I shall shew you, is in the nervous system. 



" ' All sensations of impression' I am doubtful if I should 

 not have said, all sensations whether of impression or of con- 

 sciousness. But there are sensations of consciousness which are 



