264 LIFE ■' OUTLINES OF GENERAL BIOLOGY 



to persistent gentle stimuli from the central nervous system. In 

 this state there is no increase of chemical expenditure, as there is in 

 ordinary' contraction, and there is no fatigue. Now in animal 

 hypnosis there is a marked change in tliis tonus, either an increase 

 of its intensity (hypertonic), as in Arthropods, or a decrease of its 

 intensity (hj'potonic), as in most hypnotisable vertebrates. 



Some day it will be possible to make a scientific inclined plane 

 including the psychical hjT^nosis in man (and probably in a few 

 mammals), the various grades of artificial and natural animal 

 hj'pnosis, both hj'potonic and hj^pertonic, till we touch bottom in 

 the shock cramp of very simple creatures like brittle-stars. \\v 

 have probably to do with a reflex that has been repeatedly seized 

 upon and made of great service in life-preserving — sometimes even 

 in life-continuance. 



NATURE OF SLEEP 



Except to habitual dreamers, sleep is a practically uninteresting 

 state, a blank dies non, that recurs over a large fraction — about a 

 third — of our life! One grudges the time one spends in sleep, and it 

 is one of the consolations of ageing that for a long time we require 

 less sleep. Yet we should not like to be guinea-pigs, which are said 

 not to sleep at all. 



But while sleep is practically uninteresting, it is very different 

 when we think of it theoretically. It is a brain-stretching problem, 

 full of psycho-biological interest. Thus the number of theories about 

 sleep is huge. One recalls the crowd of theories in regard to sex, 

 another everyday phenomenon whose true inwardness is still 

 unknown or uncertain. 



The problem need not be complicated by using the word "sleep" 

 in a loose way. Thus hibernation is certainly not sleep, nor is 

 coma, nor trance. The "sleep" of plants is a misnomer, and while 

 we may have our opinions, we have no certainty as to the occurrence 

 of true sleep below the level of mammals and birds. True sleep is 

 part of an established rhythm, and implies a .state of partial fatigue 

 and rest in the higher nerve-centres, during which recuperation 

 occurs, perhaps in part as.sociated with the removal of inhibiting 

 waste-products or toxins. Yet it is certain that our whole nervous 

 system does not fall asleep, for the "breathing centre" in our 

 medulla must be always awake; and so is it with many parts of 

 the body, such as heart, kidneys, and alimentary canal, which 

 require some nervous control. Even our muscles are not quite 

 inactive when we are sleeping, else we should soon become cold.evTU 

 vmd(T a cjuilt. 



Familiarly wrapped up with sleep are several subsidiary features, 

 not quite essential. Thus by closing our eyes and shutting out light 



