PHYSIOLOGY OF SLEEP—LEGENDRE. 589 
Unconsciousness caused by chloroform, ether, and other anesthetics 
bears only a distant relation to natural sleep. It differs, among other 
things, in the impossibility of exciting an awakening. Coma differs 
still further, for it is very close to death; while sleeping sickness is 
comparable only in name, for it is caused by parasites and produces 
torpor only in its last stages. Laying aside all these questions, the 
exposition of which would require several lectures, I will talk to you 
of natural sleep alone, and of that only as a physiologist, for the 
physiologist also has his opinion to express upon the subject. 
Generally the physiologist hardly speaks of sleep, and though it 
takes up the third of our lives, it is far from occupying a third part 
of. physiological books. A page or a few lines is all we find about 
this subject even in the largest treatises. It 1s not only studies and 
definite theories concerning sleep that are lacking among physiolo- 
gists, but precise observations are rare because they are difficult. 
How, in fact, can you experiment upon a being wrapped in slumber 
without waking him? How can you produce genuine sleep, which 
is really a voluntary act? How distinguish natural sleep from the 
state of torpor so often produced in experiments? Though all these 
questions are not answered, yet the subject seems to me to be of 
enough interest to warrant my showing you the actual conditions of 
this great problem. Let us first of all try to understand what that 
is which is called sleep, though it is much easier to tell what it is not 
than what it really is. It is certainly not like a narcotic state, nor 
hypnotism, nor the lethargy which I just mentioned. It is different 
from the changing rhythms of certain plants, as the truffle, the sensi- 
tive plant, and it differs also from the hibernating sleep of the snail, 
the marmot, and many other animals. But what is it exactly, and 
what do we know about it? There are definitions enough by psychol- 
ogists as well as by physiologists, but none are satisfactory, and I 
believe it would be better to at once describe the mechanism of sleep 
rather than tediously to hunt for its precise formula. 
Let us then examine a man asleep, a man in preference to an animal, 
for to external observations we can add a description of what we feel 
in ourselves. Contrary to what one would be inclined to believe, we 
do not go to sleep because we are fatigued, for great fatigue may 
indeed provoke insomnia, just as a long walk in the country will cause 
great exhanstion in a person who is not in training and often ren- 
der difficult the customary slumber. We actually go to sleep 
either through habit or through a spirit of indifference to our sur- 
roundings. TLrough habit, we go to bed every evening and sleep all 
night. The sight of our bedroom, of our couch, the darkness, the 
silence, recall the habit, and incite us to slumber. But this is not 
merely a habit, for it varies with each individual. One who goes to 
bed regularly at 10 o’clock becomes sleepy each evening at that hour, 
