1154 SLEEP. [Book iv. 



tem, must, night after night, or day after day, or at least time 

 after time, lay them down to sleep. The salient feature of 

 sleep is the cessation or extreme lowering of the psychical 

 activity of the brain and of the nervous processes which serve 

 as the basis of that activity. When sleep is at its height, the 

 afferent nervous impulses which external agents set going in 

 the afferent somatic nerves such as those of the special senses, 

 are no longer the starting points of complex cerebral processes; 

 not only do they fail to excite consciousness and to leave their 

 mark on memory, but they may be unable to call forth even a 

 simple reflex movement. And yet they are not wholly without 

 effect ; for though a set of feeble afferent impulses may pro- 

 duce no visible reaction and leave no impression on the mind of 

 the sleeper, yet impulses of the same kind, if made stronger in 

 proportion to the depth of the sleep, may be followed by their 

 wonted cerebral consequences, and may thus awake, as we say, 

 the sleeper. It would seem as if the afferent impulses met in 

 their course with an unwonted resistance to their progress, as 

 if the wheels of the cerebral machinery worked stiffly so that 

 the lesser shocks of molecular change which otherwise would 

 have moved them, were broken and wasted upon them. Cor- 

 responding to this block or lessened inroad of afferent impulses, 

 the outflow of efferent impulses is stopped or largely dimin- 

 ished ; the body gives no sign of the working of a conscious 

 will, the eyelids drop and the head nods, and the various actions 

 by which the erect posture is maintained are let go for lack of 

 the governing motor impulses. And psychological self-inquiry 

 tells us that in complete sleep this absence of outward signs of 

 cerebral activity has its fellow in the absence of inward marks ; 

 the interval between falling asleep and awakening is a blank 

 and gap in the history of the mind. 



We say ' complete sleep ' since there are many degrees of 

 sleep, the state which we call that of dreaming being one of 

 them ; and between the most perfect wide-awakefulness and 

 that deepest slumber which refuses for a long time to give way 

 before even the strongest stimuli, no clear line of demarcation 

 can be drawn. When we fall asleep the tie between 'ourselves' 

 and the external world is not suddenly snapped, we do not by 

 one step pass from consciousness to unconsciousness ; and the 

 same when conversely we awake ; as the world vanishes from 

 us or comes back to us, the afferent impulses of sight, of sound 

 and of other kinds, for a period which may be brief but always 

 exists, produce, before they cease or begin appreciabty to affect 

 us at all, effects in ascending or descending scale which we call 

 unreal. And the outward signs of sleep may vary from one in 

 which volition is present and even dominant, to one in which even 

 the simplest reflex movements of the skeletal muscles are with 

 difficulty evoked, and the maintenance of some skeletal tone 



