31 6 THE GROWTH OF THE BRAIN. 



applied to the cranial nerves the sleeper is apt to be 

 recalled to consciousness by the redistribution of his 

 blood, the supply to the encephalon being increased. 1 

 Kohlschiittcr has reported observations on the depth of 

 sleep, which was measured by the height from which 

 a falling ball must drop in order to make a sound suffi- 

 ciently loud to waken the sleeper. 



The ease with which the subject awakens is mani- 

 festly not a measure of recuperation, since after two 

 and a half hours of sleep a very slight stimulus is 

 efficient. A condition permitting the diffusion of the 

 auditory stimuli is therefore established long before the 

 reconstructive changes in the cells in general are com- 

 plete, and there is reason to think that the first change 

 is due to an increased supply of blood to the central 

 system, after which the constructive processes slowly 

 follow in the cells. For this last, a period of four hours 

 or more is usually demanded, and when less time 

 is allowed, the cells return to work under great 

 disadvantages. 



The feat of walking one thousand miles in a thousand 

 hours, at the rate of a mile an hour, leaves the pedes- 

 trian with at least two-thirds of each hour unoccupied, 

 yet it requires the greatest endurance, from the fact of the 

 discontinuity of sleep ; not only, therefore, the amount 

 of time taken for sleep, but the length of the single 

 periods, is significant. Continuous loss of sleep is far 

 more rapidly fatal than starvation, and the final changes 

 are very marked, especially in the nervous system. 2 



These physiological phenomena are correlated with 

 the anatomical variations found in the cell elements at 

 different times. The changes which occur in the bodies 

 of nerve cells as the result of their activity have been 



1 Kohlschiitter, Zdtschr.f. rat. Med., 1863. 



2 De Manaceine, Arch. Italienne de Bwl., 1894. 



