41 6 THE POPULAR SCIENCE MONTHLY. 



similar view, and the general conclusion now is, that during sleep the 

 brain is really supplied with less blood than in waking hours. 



To account for the reason why the brain is less freely fed with 

 blood in sleep, it has been surmised that the vessels, the arteries, which 

 feed the brain, and which for contractile purposes are supplied with 

 nerves from the organic nervous system, are, under their nervous in- 

 fluence, made to close so that a portion at least of the blood which 

 enters through them is cut off on going to sleep. This view, however, 

 presupposes that the organic nervous centres, instead of sharing in the 

 exhaustion incident to labor, put forth increased power after fatigue, 

 an idea incompatible with all we know of the natural functions. 



Carmichael, an excellent physiologist, thought that sleep was 

 brought on by a change in the assimilation of the brain, and by what 

 he called the deposition of new matter in the organ, but he offered no 

 evidence in proof: while Metcalfe, one of the most learned physicists 

 and physicians of our time, maintained that the proximate cause of 

 sleep is an expenditure of the substance and vital energy of the brain, 

 nerves, and voluntary muscles, beyond what they receive when awake, 

 and that the specific office of sleep is the restoration of what has been 

 wasted by exercise : the most remarkable difference between exercise 

 and sleep being, that during exercise the expenditure exceeds the 

 income ; whereas during sleep the income exceeds the expenditure. 

 This idea of Metcalfe's expresses, probably, a broad truth, but it is too 

 general to indicate the proximate cause of sleep, to explain which is 

 the object of his proposition. 



My own researches on the proximate cause of sleep researches 

 which of late years have been steadily pursued lead me to the conclu- 

 sion that none of the theories as yet offered account correctly for the 

 natural phenomenon of sleep ; although I must express that some of 

 them are based on well-defined facts. It is perfectly true that exhaus- 

 tion of the brain will induce phenomena so closely allied to the phe- 

 nomena of natural sleep, that no one could tell the artificially-induced 

 from the natural sleep ; and it is equally true that pressure upon the 

 brain will also lead to a state of sleep simulating the natural. For 

 example, in a young animal, a pigeon, I can induce the deepest sleep 

 by exposing the brain to the influence of extreme cold. I have had a 

 bird sleeping calmly for ten hours under the local influence of cold. 

 During this time the state of the brain is one of extreme blood lessness, 

 and, when the cold is cautiously withdrawn and the brain is allowed to 

 refill gently with blood, the sleep passes away. This is clear enough, 

 and the cold, it may be urged, produces contraction of the brain-sub- 

 stance and of the vessels, with diminution of blood, and with sleep as 

 the result. But if, when the animal is awaking from this sleep induced 

 by cold, I apply warmth, for the unsealing of the parts, a little too 

 freely, if, that is to say, I restore the natural warmth too quickly, then 

 the animal falls asleep again under an opposite condition ; for now into 



