PHYSIOLOGY. 121 



fluids ; for we have now many instances of the sleep induced by 

 cold, where the waking state was quickly recoverable by mode- 

 rate stimuli., so that there was no suspicion that the fluids were 

 coagulated ; and, upon the contrary, there are experiments upon 

 such animals subjected to cold, which induced sleep ending in 

 death, in which the fluids appeared to be in their common flu- 

 idity, and in motion as before, though perhaps more languidly 

 and not equally extensively. I must further notice, that in 

 animals which seem dead during the winter, as bats and swal- 

 lows, I imagine that their fluids are at no time frozen, but that 

 they merely stagnate when the motion is destroyed, and take 

 their ordinary coagulation : indeed all these animals retreat to 

 places to which the freezing cold never reaches, and they collect 

 in such numbers that the mass is never affected to the freezing 

 point. From the whole history of animal life constantly ex- 

 cited, and from its first beginnings reared to a certain degree of 

 vigour, by a certain degree of heat, and from the check which 

 cold produces on these risings of life, we can scarcely doubt 

 that as heat produces life, by exciting the activity of the nerv- 

 ous system, so cold, its counterpart, induces a state of sleep by 

 encroaching upon the same power. 



" 6 The absence of impressions. ' Lay a man upon a soft bed, 

 remove the stimuli of light and noise, and if he does not think 

 much, he will fall asleep at any hour of the day. The fact 

 will not be refused ; and the exceptions are, where the mind is 

 put into a train of activity. Impressions being the beginning, 

 are also the chief support of the activity of the whole of our 

 nervous system ; and it^is from thence that all our sensations, 

 and the actions depending upon them, do arise. It is neces- 

 sary to this activity that these impressions should continue ; on 

 withdrawing, therefore, these causes of motion, the motion sub- 

 sides, and sleep ensues. 



u ' Attention to a single sensation."* This is a matter of 

 common observation, that the hum of bees, the murmur of a dis- 

 tant stream, the lullaby of a nurse, will set a man asleep, as is 

 finely pointed out in the lines of the poet : 



" Hyblacis apibus florcm depasta salicti 

 Sacpc levi somnum suadebit inire susurro." 



VIRG. EC. I. v. 55. 



