70 "klKKI' and nitl'IAMri." 



Anotlior liy])oi-,liosiK as to the immcdiiito canso of sloop is 

 his: tliat as the novvos aot tluiy undcii'i^-o chtimioal cha,iin'o, 

 and the wasto pTodncts of tlioir activity accumulate diii'ing 

 the day and clog their action. This is an insnfficient expla- 

 nation on various grounds, amongst others, that those waste 

 products do not probably accumulate more in tho bi'ain 

 than in other parts, such as the heart muscle, and there is 

 no particular reason why they should be removed by sloop. 

 Another undoubted reason of sloop is tho most certain, viz. : 

 tho mere exhaustion of the norvo cells. No pai't of tho 

 body can go on for ever, and some parts require i/onewing 

 or resting more often than others; the norvo cells appear 

 to require nearly a third of their time to re-create them- 

 selves. I tln'nk we may give the three main causes of sleep, 

 therefore, as follows : — (1) Exhaustion of the nerve colls ; 

 (2) withdrawal of those stimuli fi'om tho norvo cen),rcs 

 which, tend to keep them awalce ; and Qi) inherited habit. 



Amongst tho many interesting en(.|nirics that have 

 occurred to mo whilst thinking about the subject, 1 vvill 

 confine myself to two or three. 



(1) As to the average quantity of shop. This is, to a 

 very great extent, a question of habit. Any one who sleeps 

 nine or ten hours out of the twenty-four, fools drowsy if 

 suddenly obliged to take only eight. If an ordinary man 

 accustoms himself to six or seven, ho will be as active 

 during the day as if lu! took eight or nine. Individual peca- 

 liarity, no doubt, makes a difference ; quick bi'ains as a I'ulo, 

 I think, require loss than slow ones. 



Tho accounts of saints and hermits, who only idlowod 

 themselves four or five hours a night, are quite oredible ; in 

 fact, the more sleep that is taken over and above sovon hours 

 I)robably diminishes the activities of tho daytime. It is 

 stated that men who are capable of long continuous mental 



