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 m 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. 



GeneraUy 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 

 tins subject even in the largest treatises. It is 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 exhaustion 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. Through habit, we go to bed every evening and sleep all 

 night. The sight of our bedroom, of our couch, the darkness, the 

 silence, recaU 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, 



