THE WONDER OF LIFE 495 



Animals accustomed to sleep wiU die in a few days if 

 they are deprived of it ; in some cases, much sooner, as 

 has been shown experimentally, than if they are deprived 

 of food. What, then, is it that goes on during sleep that 

 makes it necessary ? What is the physiological condition 

 during sleep ? 



In a recent lecture, Legendre summed up the state of 

 afEairs : — ^Digestion goes on, and this may lessen the blood- 

 supply to the brain ; perspiration and excretion go on ; 

 the respiratory movements are altered, being usually 

 slower, deeper, and more regular ; relatively less carbonic 

 acid is given off ; the body temperature falls ; the action 

 of the heart is slowed and the arterial blood pressure 

 diminishes ; there is a relative anemia of the brain ; the 

 working of the sense-organs is altered ; the muscles are 

 generally relaxed and the reflexes tend to disappear ; 

 and there are other differences of a subtler sort. But it 

 cannot be said that there is in this narration anything 

 that in particular gives us the clue to the signiiicance of 

 sleep. 



There is a superabundance of theories in regard to the 

 cause of sleep, but, until recently, it was the dehght of 

 physiologists to show that none of them was adequate. 

 It has been suggested that a relative ansemia of the brain 

 left the nerve-cells without enough of food or encumbered 

 with imperfectly removed waste. It has been suggested 

 that changes in the condition of the blood produced sleep. 

 It has been suggested that wearied nerve-cells contracted 

 and lost that touch with one another that they have during 

 waking hours. It has been suggested that the creature 

 becomes irresponsive and indiSerent to the outside stimuli 

 that keep it agog in its waking hours. But, as Claparede 



