OX SLEEP. 
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which the skull has been injured and an artificial opening 
through it to the brain has been formed, pressure upon the ex- 
posed surface has led to a comatose condition. I once myself 
saw a case of this nature. But the evidence against this ex- 
planation is strong, because the sleeping brain has been observed 
to be pale and too free of blood to convey any idea of pressure. 
In opposition to the pressure theory, Blumenbacli contended 
that sleep is due to a diminished flow and impulse of blood 
upon the brain, for he argued the phenomenon of sleep is in- 
duced by exhaustion, and particularly by exhaustion following 
upon direct loss of blood. Recently Mr. Arthur Durham, in a 
very able communication, has adduced a similar view, and the 
general conclusion now is that during sleep the brain is really 
supplied with less blood than in waking horns. 
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 pur- 
poses are supplied with nerves from the organic nervous system, 
are, under their nervous influence, made to close so that a por- 
tion 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 labour, 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, main- 
tained that the proximate cause of sleep is an expenditure of 
the substance and vital energy of the brain, nerves, and volun- 
tary 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 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 — re- 
i searches which of late years have been steadily pursued — lead 
me to the conclusion that none of the theories as yet offered ac- 
count 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 exhaustion of the brain will 
induce phenomena so closely allied to the phenomena of 
natural sleep, that no one could tell the artificially induced from 
