SLEEP 163 



It is curious how incomplete at present is the physio- 

 logist's knowledge of both the actual condition of the 

 brain in sleep and of the immediate causes which produce 

 that condition. It is probably true (though it is dis- 

 puted) that the brain becomes pale during sleep, owing 

 to a contraction of the blood vessels, and that the in- 

 activity of the brain arises from this condition. But it is 

 not obvious what determines the contraction of these 

 vessels at the definitely recurring period of sleep. It is 

 probable that the nervous tissue of the brain is, as are 

 the muscles of the body, poisoned or choked (as it were) 

 by the chemical products of the day's activity, and so 

 readily cease to be active until the injurious products 

 have had time to be carried away by the blood stream. 

 Muscular substance undoubtedly is affected in this way, 

 and that great muscle the heart, though never resting for 

 a lengthened period, rests after each pulse or contraction, 

 and recovers itself in the brief interval. 



It is also probable that the exhaustion by the day's 

 activity of the oxygen stored up in the various tissues of 

 the body produces a condition of quiescence whilst the 

 store is replenished. Stimulation of the nerves through 

 the sense-organs of sight, hearing, and touch will prevent 

 or retard this natural quiescence, and the cessation of 

 that stimulation is favoured first of all by the darkness of 

 night and by the closing of the eyelid, as well as by the 

 removal of clothes which more or less irritate the skin ; 

 also by the would-be sleeper taking up a position of 

 perfect rest, and by the exercise of his will, withdrawing 

 his brain as much as possible from all external influences. 

 The would-be sleeper also controls, when possible, that 

 internal stimulation of the brain which we call attention. 

 It is the failure (owing to unhealthy conditions) to 

 control the latter which leads to the most serious kind of 

 sleeplessness, when the brain gets for hours out of re- 



