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 drawing 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 anaemia 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. Claparede 

 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 anaemia or hyper- 

 emia? 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 



