CHAP, v.] THE PHASES OF LIFE. 693 



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 protoplasmic activity when they accumulate within 

 the protoplasm 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. Indeed lactic acid, the increase of which 

 was supposed to be the cause of the acid reaction of muscular 

 and nervous tissues after exercise, has been especially pointed to 

 in this connection; but, as we have seen, the acid reaction in 

 question appears not to be due to any increased production of 

 lactic acid. Besides, if the 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 may be said in favour of 

 the conception that during the waking hours the expenditure of 

 oxygen exceeds 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, 

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

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

 inhibitory 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 

 a complication indicates that the explanation is, at present at 

 least, inadequate. 



The phenomena of sleep shew very clearly to how large an 

 extent an apparent automatism is the ultimate outcome of the 

 effects of antecedent stimulation. When we wish to go to sleep we 

 withdraw our automatic brain as much as possible from the influ- 



