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POPULAR SCIENCE REVIEW. 
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 bloodlessness, 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cautiously withdrawn and the brain is allowed to refill gently 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 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 pres- 
sure, and there is sleep. 
The same series of phenomena from opposite conditions can 
be induced by narcotic vapours. 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 engorged 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 con- 
ditions, 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 is 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 insensible — 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 
