438 POPULAR SCIENCE MONTHLY. 



the tide of nervous impulses pours with intensified energy through the 

 narrowed outlets remaining — an idea borrowed from Lugaro. If we 

 consider that a man is most thoroughly awake when his attention is 

 most rigidly concentrated, when he is a 'man of one idea/ we shall 

 perhaps incline toward Lugaro 's conception of sleep, which is cer- 

 tainly as far as possible removed from this mental fixedness. Hyp- 

 nosis is accompanied by cerebral congestion and natural sleep by 

 anemia. There is accordingly a strong temptation to suppose that 

 the cell-changes in the two states are opposite in their nature, that 

 in hypnosis the retraction of the dendrites is characteristic and in 

 natural sleep their extension. The sluggish condition of the mind 

 under suggestion as compared with its fanciful fiights in dreaming 

 falls happily in line with this view. But such speculation is pre- 

 mature. 



It was said at the outset that the several theories of sleep are not 

 all mutually exclusive. It is possible to go beyond this statement, 

 for we may assign a place to each of those mentioned without incon- 

 sistency. We may suppose in the first place that the alternation of 

 day and night through the ages has impressed its rhythm upon the 

 race, so that it is hard for the individual to break from the habitual 

 course in which activity is associated with light and rest with dark- 

 ness. In other words, the amount of the metabolism tends to keep 

 above a mean for some hours and then to fall below it. The excess 

 of destructive processes over those which are recuperative during the 

 waking hours results in general and local fatigue, a condition into 

 which may enter both the depletion of intra-molecular oxygen and the 

 accumulation of toxic waste-products. While this progressive loss of 

 condition affects the body as a whole, the nervous system is subject to 

 its own peculiar drains. It is very probably the hard-worked vaso- 

 motor center which proves to be the vulnerable spot. With its release 

 of the blood-vessels in certain areas from its reenforeing influence 

 comes the cerebral anemia. Then, we may suppose, the nerve-cells 

 become less active than in the brain which has its full supply of blood, 

 that they cease to send impulses over the usual routes, either because 

 gaps have opened or because such impulses as do arise are permitted 

 to stray and be scattered, producing no effect in consciousness or one 

 which is quite bizarre and meaningless. 



Such an outline as this is a composite scheme in which the con- 

 ditions emphasized by Pfliiger and Preyer are given recognition as 

 fundamental causes of sleep, Howell's idea is accepted as explaining 

 well its onset, its varying depth and the awakening, while the pictures 

 sketched by Duval and Lugaro arc combined to represent the intimate 

 state of the slumbering brain. 



