CHAP. TIL] THE PHASES OF LIFE. 415 



Both, though quiescent, are irritable, in both the quiescence will 

 ultimately give place to activity, and in both an appropriate 

 stimulus applied at the right time will determine the change from 

 rest to action. Just as a single prick will under certain circum- 

 stances awake a ventricle, which for some seconds has been 

 motionless, into a rhythmic activity of many beats, so a loud noise 

 will start a man from sleep into a long day's wakefulness. And 

 just as in the heart the cardiac irritability is lowest at the beginning 

 of the diastole and increases onwards till a beat bursts out, so is 

 sleep deepest at its commencement after the day's labour ; thence 

 onward slighter and slighter stimuli are needed to wake the sleeper. 

 For judging of the depth of ordinary nocturnal sleep by the inten- 

 sity of the noise required to wake the sleeper, it may be concluded 

 that, increasing very rapidly at first, it reaches its maximum within 

 the first hour; from thence it diminishes, at first rapidly, but after- 

 wards more slowly. 



We cannot, however, at present make any definite statements 

 concerning the nature of the molecular changes which determine 

 this rhythmic rise and fall of cerebral irritability. The fact that 

 the products of metabolic activity when they accumulate within 

 a tissue appear to become in the end an obstruction to that 

 activity, has suggested the idea that the presence in the cerebral 

 tissue of an excess of the products of nervous metabolism is the 

 cause of sleep ; and a parallel has been drawn between the sleep 

 of cerebral tissue and the diminished irritability of muscular tissue 

 attending muscular fatigue, in which the products of muscular 

 metabolism have been supposed ( 86) to play an important part. 

 Indeed lactic acid has been especially pointed to in this connec- 

 tion ; but there is no solid reason for attributing any such import- 

 ance to this particular substance ; and if during the rest of sleep 

 this or any other metabolic product is washed out of the nervous 

 tissue by the blood stream we should expect a greater, not a less 

 supply of blood to the brain during sleep. Besides, if the mere 

 accumulation of metabolic products of any kind were the cause 

 of sleep, it is not clear why we should ever have any hope of 

 waking. More perhaps may be said in favour of the conception 

 that during the waking hours the expenditure of oxygen ex- 

 ceeds the income and that the quiescence, which we call sleep, 

 comes from the exhaustion of the body's store of oxygen, more 

 especially of that ' intramolecular ' oxygen of which we spoke ( 358), 

 in dealing with the respiration of the tissues. But to this view 

 must be added some hypothesis, such as the byplay of some in- 

 hibitory mechanism, whereby the respiratory centre is not roused 

 to increased activity by this lack of oxygen (for as we have seen 

 the breathing shares in the slumber of the body) though continuing 

 to play with an amount of energy, which permits a gradual 

 restoration of the lost store of oxygen and so finally brings on 

 the awakening which ends the sleep. And the necessity for such 



