436 • POPULAR SCIENCE MONTHLY. 



ening toward morning. Many people will agree that their own sensa- 

 tions 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 sug- 

 gestion. 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 mi- 

 croscopical 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 conscious- 

 ness 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 im- 

 passable, 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 conscious- 

 ness, for, in utter isolation, these cells could no longer combine their 

 activities into one whole such as forms the physical basis of psychic 



