862 THE PHASES OF LIFE. 



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 meta- 

 bolic 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 favor 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 " intra- 

 molecular " 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 breath- 

 ing 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 show 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 influence of all extrinsic stimuli ; and an in- 

 teresting case is recorded of a lad whose connection with the external world 

 was, from a complicated anaesthesia, limited to that afforded by a single eye 

 and a single ear, and who could be sent to sleep at will by closing the eye 

 and stopping the ear. 



820. The cycle of the day is, however, manifested in many other ways 

 than by the alternation of sleeping and waking, with all the indirect effects 

 of these two conditions. There is a diurnal curve of temperature (see p. 537), 

 apparently independent of all immediate circumstances, the hereditary im- 

 press of a long and ancient sequence of days and nights. Even the pulse, so 

 sensitive to all bodily changes, shows, running through all the immediate 

 effects of the changes of the minute and the hour, the working of a diurnal 

 influence which cannot be accounted for by waking and sleeping, by work- 

 ing and resting, by meals and abstinence* between meals. And the same 

 may be said concerning the rhythm of respiration, and the products of pul- 

 monary, cutaneous, and urinary excretion. There seems to be a daily curve 

 of bodily metabolism, which is not the product of the day's events. Within 

 the day we have the narrower rhythm of the respiratory centre with the 

 accompanying rise and fall of activity in the vasomotor centres. And 

 lastly, there stands out the fundamental fact of all bodily periodicity, that 

 alternation of the heart's systole and diastole which ceases only at death. 

 Though, as we have seen, the intermittent flow in the arteries is toned down 

 in the capillaries to an apparently continuous flow, still the constantly re- 

 peated cycle of the cardiac shuttle must leave its mark throughout the 

 whole web of the body's life. 



