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 certainly as far as possible removed from this mental fixedness. Hypnosis 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 flights in dreaming falls happily in line with this view. But such speculation is premature.
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 inconsistency. 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 darkness. 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 vasomotor 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 conditions emphasized by Pflüger 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.
