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POPULAR SCIENCE REVIEW. 
say, as Blumenbach has said, that in this state all intercourse 
between mind and body is suspended, is more perhaps than 
should be said, the precise limits and connections of mind and 
body being unknown. But certainly the brain and spinal cord, 
ceasing themselves to receive impressions, cease to communi- 
cate to the muscles they supply stimulus for motion, and the 
muscles under their control with their nerves therefore sleep. 
And so, to the extent that the acts of the brain and cord and 
their nerves are mental, and the acts or motions of the volun- 
tary muscles are bodily acts ; to that extent, in sleep, the inter- 
course between the mind and the body is suspended. 
Tiie Physical Cause of Sleep. 
In sleep the condition of the voluntary muscles and of the 
voluntary nervous system is, we must assume, in some manner 
modified, since these organs are transformed from the active into 
the passive state. Respecting the condition of the muscles in 
sleep, no study of a systematic sort has been carried out, but in 
relation to the brain there has been much thoughtful study, 
upon which many theories have been founded. 
The older physiologists regarded sleep as due to the exhaustion 
of the nervous fluid ; during sleep, they held, this fluid accu- 
mulates in the brain ; and, when the brain and the other centres 
and nerves of the cerebro-spinal system are, to employ a common 
expression, recharged, the muscles are stimulated and the body 
awakes ; the brain prepared to receive external impressions and 
to animate the muscles, and the muscles renovated and ready 
to be recalled into activity. This theory held its ground for 
many years, and, perhaps, still there are more believers in 
it than in any other. It fails to convince the sceptical be- 
cause of its incompleteness, for it tells nothing about the nature 
of the presumed nervous fluid, and we know nothing as yet 
about this fluid. The primary step of the speculation is con- 
sequently itself purely hypothetical. 
Another theory, that has been promulgated, is that sleep 
depends on the sinking or collapse of the laminae of the cere- 
bellum or little brain. This theory is based on the experiment 
that compression of the cerebellum induces sleep; but the 
argument is fallacious, because pressure on the larger brain, or 
cerebrum, is followed by the same result. The theory of pres- 
sure has been proposed again in a different way ; it has been 
affirmed that the phenomenon of sleep is caused by the accu- 
mulation of fluids in the cavity of the cranium, and by pressure, 
resulting from this accumulation, on the brain as a whole. We 
know well that pressure upon the brain does lead to an in- 
sensible condition resembling sleep, and in some instances, in 
