266 LIFE : OUTLINES OF GENERAL BIOLOGY 



affects electrical conductivity between each side of the membrane; 

 and there seems to be an increase in the electrical resistance of the 

 body generally during sleep. This, at any rate, is a vigorous theory 

 of to-day, that a centre in the floor of the brain is the seat of physio- 

 logical processes which initiate, maintain, and abolish sleep. 



But this theory, as Dr. Gillespie shows, can be made more cir- 

 cumstantial. Pavlov and others have proved that prolonged stimu- 

 lation, followed by responses, may bring about in the impHcated 

 nerve-cells an inhibitory state which prevents further activity for 

 the time being. This has a protective value, for it prevents 

 excessive fatigue of the brain. Now it may be that sleep is due to 

 an irradiation of inhibitory influences from a focus gradually evolved 

 for that function, and situated apparently in the thalamic region 

 of the brain. The irradiation of the inhibitory commands may be 

 brought about by over-stimulation and consequent fatigue, but also 

 by a shutting out of stimuli; and account must also be taken of 

 established rhythms and habituations. For all these problems there 

 are two approaches — the physiological and the psychological. The 

 former leads us to think of changes in the calcium metabolism, in 

 the permeability of cell membranes, in electric potential, and so 

 forth. The other leads us to think of a safeguard that prevents the 

 restless mind from overtiring the weak flesh. Sleep is a tax on keen 

 wits. And that, dear reader, is why you are so often sleepy! The 

 more wideawake you are, intellectually and artistically, the more 

 sleep you require. There is a warning to man in the guinea-pig that 

 does not sleep! 



Sleeplessness. — It is very doubtful if true sleep ever occurs 

 except in warm-blooded animals (birds and mammals), and it does 

 not seem to occur in all of these. Sleep is an established bodily habit 

 or rhythm, which brings about a quieting down of activities, and, 

 as we have said, it is probably regulated automatically by a special 

 centre in the brain, just like the heat-regulating centre which keeps 

 the temperature of our body practically constant, day and night. 

 Sleep puts on a brake over the whole bod}^- it inhibits the usual 

 answers-back to stimuli; it shuts the doors and draws down the 

 blinds; it acts as a safeguard against the fatigue-collapse which is 

 apt to follow the often-repeated response to the same kind of excite- 

 ment. A certain amount of fatigue may be useful in prompting the 

 sleep centre to exert its beneficent life-saving influence in enforcing 

 rest, but the favourite theory that sleep is a self- intoxication of 

 the body and brain with the fatigue-toxins of prolonged activity 

 is far too simple. Many people know what it is to be too tired to 

 sleep; many sleep very soundly who never know what it is to be 

 tired. 



While it seems wisest to regard the true inwardness of sleep as 

 still a problem, if not a puzzle, we know a ^ood deal about it? 



