282 THE POPULAR SCIENCE MONTHLY. 



"When studying the physiology of nerve-action as we see it in 

 animals, children, and men, and the pathology as we see it in various 

 nervous disorders, as acute insanity, delirium tremens, and the like, 

 we soon become aware of the fact that the well-being of the entire 

 nervous system depends mainly on its renovation during a state of 

 repose ; and that for the higher portion of the brain, at any rate, this 

 state of repose and rest is synonymous with healthy sleep. Round 

 the phenomena of sleep, and its causes and conditions, are grouped 

 many of the problems which have to be solved by the physiologist 

 who has to investigate the action of the nerve-centres, and the physi- 

 cian who has to cure their disorders. The state or condition of a 

 nerve-centre, which I have called the force, will be dependent upon 

 the amount of rest and sleep which it enjoys, supplemented by two 

 other restorers of force, food and warmth ; which must also be taken 

 into consideration. 



Observation teaches us that all animals sleep after a certain period 

 of bodily fatigue, which varies according to the individual, the young 

 requiring sleep more quickly than the old, and a larger amount. If 

 the fatigue be great, nothing can keep a child or even a man awake. 

 When refreshed by sleep, when the force is again accumulated in the 

 brain, we wake spontaneously, or are awakened by trifling stimuli, 

 as sounds or light. This alternation of sleep and waking is the normal 

 state of health, and absence of sleep is something abnormal : it is a 

 disorder, and must lead to further disorder if "prolonged. Sleep is not 

 necessary for the renewal of force in every centre. In very severe 

 muscular exercise mere cessation for a time recruits our force, and 

 euables us to begin again ; but for the higher work of the brain sleep 

 is indispensable, and all brain- work, and indeed life itself, must cease, 

 unless by this the force is renewed. 



So much does observation teach us of the reparation of the force 

 of the brain during sleep. Experiment, however, enables us to state 

 the physiological condition of the brain in sleep, and so to analyze 

 further the production and expenditure of this nerve-force. In sleep, 

 as we have seen, it is produced and accumulated ; in active waking 

 hours it is expended. In sleep, the arterial circulation of the brain 

 falls to a certain point, and metamorphosis consequently is reduced to 

 a minimum. When the brain is acting, evfen in dreams, the blood-flow 

 increases both in arteries and veins. To promote sleep, we seek to 

 diminish this arterial current; until this is done, sleep comes not. 

 The two things which chiefly produce sleep in a healthy man or ani- 

 mal are fatigue and food. After a hearty meal, or after great fatigue 

 endured for many waking hours, it will be difficult to rouse him from 

 sleep, and when roused he will be torpid and inactive, and will fall 

 back into sleep easily. His brain will be emptied of blood, and ordi- 

 nary stimuli, as light, sound, and movement of others, will not bring 

 back the blood to his brain: moreover, his blood will contain less 



