THE PHYSIOLOGY OF SLEEP. 41 7 



the relaxed vessels of the brain the heart injects blood so freely, that 

 the vessels, in like manner as when the frozen hand is held near the 

 fire, become engorged with blood, there is congestion, there is pressure, 

 and there is sleep. 



The same series of phenomena from opposite conditions can be 

 induced by narcotic vapors. There is a fluid called chloride of aonyl, 

 which, by inhalation, causes the deepest sleep ; during the sleep so 

 induced, the brain is as bloodless as if it were frozen. There is an 

 ether called methylic, which, by inhalation, can be made to produce 

 the deepest sleep ; during this sleep the vessels of the brain are en- 

 goi-ged with blood. 



We are therefore correct in supposing that artificial sleep may be 

 induced both by removal of blood from the brain, and by pressure of 

 blood upon the brain ; and in the facts there is, when we consider them, 

 nothing extraordinary. In both conditions, the natural state of the 

 brain is altered ; it cannot, under either state, properly receive or 

 transmit motion ; so it is quiescent, it sleeps. The experimental proof 

 of this can be performed on any part of the body where there are nerve- 

 fibre and blood-vessel ; if I freeze a portion of my skin, by ether-spray, 

 I make it insensible to all impression I make it sleep ; if I place over 

 a portion of skin a cupping-tube, and forcibly induce intense congestion 

 of vessels, by exhausting the air of the tube, I make the part also in- 

 sensible I make it sleep. 



The two most plausible theories of sleep the plenum and the 

 vacuum theories I had nearly called them are, then, based on facts ; 

 but still I think them fallacious. The theory that natural sleep depends 

 on pressure of the brain from blood is disproved by the observations 

 that have been made of the brain during sleep, while the mechanism 

 of the circulation through the brain furnishes no thought of this theory 

 as being possibly correct. The theory that sleep is caused by with- 

 drawal of blood from the brain, by contraction of its arterial vessels, is 

 disproved by many considerations. It presupposes that at the time 

 when the cerebro-spmal nervous system is most wearied the organic 

 system is most active ; and it assumes that the great volume of blood 

 which circulates through the brain can be cut off without evidence of 

 increased volume of blood and tension of vessel in other parts of the 

 body, a supposition directly negatived by the actual experiment of 

 cutting: off the blood from the brain. 



There is another potent objection applicable to both theories. When 

 sleep is artificially induced, either by subjecting the brain to pressure 

 of blood, or to exhaustion of blood, the sleep is of such a kind that the 

 sleeper cannot be roused until the influence at work to produce the 

 sleep is removed. But, in natural sleep, the sleeper can always be 

 roused by motion or vibration. We call to a person supposed to be 

 sleeping naturally, or we shake him, and if we cannot rouse him 

 we know there is danger; but how could these simple acts remove 

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