THE WONDER OF LIFE 499 



It appears to us probable, as we have hinted, that there are 

 levels in the animal kingdom at which the purely physio- 

 logical theory of sleep is adequate to cover all the facts. All 

 sleep is not the same sleep, any more than all flesh is the 

 same flesh. 



Claparede objects to the purely physiological view on 

 various grounds. One can sleep without being fatigued, 

 and one may be too tired to sleep. If sleep were enforced 

 by the accumulation of fatigue-poisons, how is it that 

 many a man is so lively just a few minutes before he goes 

 to bed ? Could an auto-intoxication of the severity sug- 

 gested be endured night after night for threescore years and 

 ten ? One of the Siamese twins could sleep while the other 

 suffered from insomnia, yet their blood-vessels communi- 

 cated ! Perhaps there are answers to these objections, 

 but we shall not go into that. Our point is simply to show 

 that there are great difficulties in the way of the purely 

 physiological theory. Claparede maintains that an adequate 

 theory must be psychological as well. 



Experiments made by Legendre and Pieron confirm 

 the theory that specific waste-products or fatigue-toxins 

 are formed during periods of prolonged wakefulness, that 

 these permeate the organism, and particularly affect the 

 frontal lobes of the brain. They prevented dogs from 

 sleeping, while tiring them as little as possible, and found 

 that about ten days was the limit of resistance. 



' The temperature of the body remains normal, the res- 

 piration undergoes no variation, and the amount of carbon 

 dioxide in the blood does not increase, which enables us 

 to exclude the theories of the impoverishment of the blood 

 in oxygen and its enrichment in carbon dioxide as the actual 

 causes of sleep. Neither the blood nor the brain lose their 



