596 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



us to examine its definitions, and actual experiments permit a dis- 

 cussion of the theories which aim to explain it. 



What causes sleep ? Why do we sleep ? An answer is difficult 

 when we consider all the phenomena that we shall enumerate. If 

 you aim to be precise there is great risk of being either inexact or 

 incomplete, and on the other hand if you wish to make your statement 

 general it will probably lack precision. 



I hope that what I have pointed out may indicate the difficulties 

 involved in an explanation of this phenomenon. To one who has 

 observed and reflected but little, the explanation is very simple, for 

 he easily imagines that such and such a cause induces sleep. But 

 when the observations are accumulated, since their examination 

 requires a careful study of all their phases, then the answer to the 

 problem becomes difficult, and one is never certain of solving it, even 

 after long research. It is in this spirit that I would like to have you 

 listen to the conclusion of this lecture. The inconsistency existing 

 between learned men sometimes brings a smile to the lips of those who 

 do not understand all the difficulties of their work; but it is due only 

 to their desire for the truth, to their thirst for progress, and I beg of 

 you to consider the theories that I am about to propound to you with 

 good will and sympathy. 



Let us consider, then, the various explanations that have been 

 given for sleep. 



Some early writers thought that sleep depends on a flow of blood to 

 the brain resulting from a recumbent position; but we have seen that 

 the brain contains less blood during sleep, and we know besides that a 

 person can lie down for a long time without sleeping. That theory, 

 therefore, antedating the experiments, has no longer any interest 

 other than one of curiosity. Repeated observations on the deplace- 

 ment of the blood pressure from the brain to the extremities during 

 sleep gave basis for the thought that sleep is due to cerebral anaemia. 

 The diminution of the quantity of blood in the brain produces sleep 

 by various mechanical actions. If, then, the brain fails to receive 

 enough nourishment or if the waste be not quickly enough removed, 

 the cerebral cells will cease to work either from anaemia or from intoxi- 

 cation. But other writers have criticised these hypotheses. Brod- 

 mann, as we have said, observed an increase of blood pressure at the 

 moment of going to sleep and, still further, he could not establish the 

 relation between the circulation in the brain and that of the extremi- 

 ties which forms the basis of these interpretations. Vulpian and 

 Brown-Sequard have already stated that the experiments which pro- 

 duce either a great anaemia or a great rush of blood to the brain do not 

 induce sleep. And Richet adds to the words of these critics that the 

 variations of pressure due to waking up or to going to sleep are much 

 less than those due to the position of the head, as shown by observa- 

 tions on pigeons. 



