Page:Popular Science Monthly Volume 63.djvu/440

436 toward morning. Many people will agree that their own sensations seem to imply such a second period of comparatively profound sleep.

What we call natural waking in the morning is usually due to some stimulus from without—light or sound—which would not have roused one from the deep sleep of midnight. But the stimulus may come from within, as from the state of certain organs or, curiously enough, from the previous resolution to wake at a certain time, which often operates with something of the compulsion of a hypnotic suggestion. Howell supposes that during sleep the nerve-cells of the vaso-motor center are gradually restored to prime condition and hour by hour become more irritable. So it is easier and easier as time passes to induce the vascular changes that involve waking. Moreover, the recuperated center resumes something of its normal tonic activity before consciousness returns, and so the final step is taken with none of that sense of violence that accompanies a sudden waking from sound sleep. The border-line is likely to be crossed and recrossed several times before the waking state is well established. When one is fairly roused mental activity and the pouring in of sensory impulses keep him from further napping.

Now what peculiar condition can be conceived to exist in the brain during the period of anemia and unconsciousness? What microscopical changes may be supposed to mark the transition from wakefulness to sleep? Oddly enough, the two hypotheses which are extant are quite opposite in character. The first, which has attracted the greater notice, is that of Duval. He has suggested that consciousness depends on the contact of cell-processes in the brain whereby effects are propagated from neurone to neurone. If sensory impulses are to alter consciousness, there must be a pathway for their passage. If a single synapse on the course of such a pathway is rendered impassable, the message from the sense-organ is lost from conscious life. If every sensory path is interrupted at any point between the periphery and the cortex, there must be insensibility as to the outside world and the state of the body. If all motor paths are likewise broken, there can be no voluntary action. If, in the third place, the association paths are also severed, there can be no synthetical processes of thought, no ideation. In short, the brain must lose its individuality by the breaking of connections between its structural elements. If we could suppose that every synapse in the central nervous system might be snapped, and impassable gaps opened between the cells wherever one had been wont to influence another, there must be an end of consciousness, for, in utter isolation, these cells could no longer combine their activities into one whole such as forms the physical basis of psychic