PHYSIOLOGY OF SLEEP—LEGENDRE. 597 
Modifications of the blood and of the lymph have likewise been con- 
sidered as causes of sleep. The blood, becoming more viscous and 
thicker, renders the working of the brain more difficult or dries up the 
nerve cells. Then, again, the lymph increases by d rawing water from 
the cells. Devaux, who held this last hypothesis, justifies it by the 
observation that the eyelids and the skin of the face are swollen after a 
heavy and prolonged sleep. Unfortunately, experiment proves that 
there is no relation whatever between the need of sleep and the condi- 
tion of the blood. 
Besides these circulatory theories there are others which explain 
sleep by nerve-phenomena. Sleep might be due to an interruption 
of communication between the hemispheres of the brain and the rest 
of the nervous system. Or it could be caused by the interruption of 
the contact of the nerve cells, as we: have already shown. Unfortu- 
nately, these theories lack experimental justification. 
The idea of explaining sleep by inhibition, that is, by a function of 
arrest of the nerve centers, has mislead many physiologists. In its 
most complete form, as held by Forel and Oskar Vogt, this theory 
could be explained as follows: Sleep is an inhibition produced by a 
cerebral anemia consecutive to the excitation of the vaso-motor cen- 
ters by certain factors such as the sight of the bed, by the coming of 
night, etc., or by a feeling of heaviness in the brain. 
Again, some writers have attributed sleep to the absence of external 
' exciting causes, basing the theory on observations of invalids under 
the control of general anesthesia who go to sleep as soon as their eyes 
are closed and their ears stopped. Unfortunately, these patients are 
in such a nervous condition that their sleep is not normal. And 
although silence and perfect quiet favor sleep, yet we have already 
seen that one can sleep in broad daylight in spite of noise. Claparéde 
has made a keen and striking criticism of these circulatory or nervous 
hypotheses. He says: 
We will confine our remarks to the following: First, the hypotheses mentioned are 
far from resting upon definite facts; many of them contradict one another; second, the 
supposed phenomena, were they real, might just as well be the results as the causes of 
sleep; and finally, the claim that these phenomena cause sleep, are the why and where- 
fore of its mechanism, remains problematical. Why that periodic anzemia or hyper- 
zemia? Why that retraction of the nerve cells? Why that restraint, that unrespon- 
siveness to external stimulation? The hypotheses presented only help to put off the 
solution of the problem. 
There are also other theories concerning sleep which explain it 
from a chemical standpoint. According to these, we sleep because we 
are tired and because our nervous system is exhausted, in order to 
recuperate our energy. During sleep, our bodies are like ‘a clock that 
has stopped while the weights are raised again,” or “like an engine 
when the fires are out and the firemen are renewing them,” etc. 
These chemical theories contain a great deal of truth, and, with 
