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 simile 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 anemia. 
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 anzemia 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 anzemia 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. 
