422 MENTAL FUNCTION 



demand the peculiar conditions of sleep for the recovery of their co- 

 ordinating energies. Meanwhile, however, every portion of the body 

 rests more or less completely, for quiet in the nervous system normally 

 implies repose in the other tissues. Did we know the nature of the ner- 

 vous impulse and of nerve-katabolism generally, explanation of sleep 

 would probably be forthcoming, or so far at least as its metabolic accom- 

 paniments are concerned. 



The metabolism of the whole body is doubtless lessened during sleep. 

 All researchers into respiration, for example, find the oxygen-carbon- 

 dioxide exchange decreased (according to Johansson, 31 per cent.), and 

 this is of course the best criterion of energetic expenditure. Most of this 

 loss probably comes from the greatly decreased action of the voluntary 

 muscles, but some is derived from the lessening of activity in the other 

 tissues. We do not know to what extent the accumulation of lactates 

 (from the muscles especially) influences the sleep-conditions in the ner- 

 vous tissue. Perhaps it has nothing to do with it, for mental fatigue 

 easily produces sleep and the metabolism in nerve is certainly very much 

 less than that in muscle. When nervous fatigue is too great and verges 

 on either acute or chronic exhaustion, sleep sometimes does not come. 

 This too well-known fact would seem to indicate that the occasion of 

 sleep is a chemical katabolic product of nervous action rather than of 

 muscular activity, since muscular exhaustion does not show the same 

 abeyance of sleep unless it be from muscle-pain. But the truth is that 

 the real immediate occasion ("cause") of sleep is still unknown. 



During sleep the heart slows its rate somewhat and the breath-rate 

 is markedly lessened, deepened, and made more regular (because 

 uninterrupted by speech, swallowing, etc.). Howell showed by means 

 of the plethysmograph that the volume of the hand and forearm 

 gradually increases from the beginning of sleep up to the end of an hour 

 or an hour and three-quarters, remains this size until about forty-five 

 minutes before waking, and then returns rather quickly to the normal. 

 The " general" blood-pressure generally sinks because of the lessened 

 heart-activity and the dilatation of the peripheral (and intestinal?) capil- 

 laries. It is likely enough that this abdominal and peripheral dilatation 

 is for the purpose of reducing the blood-supply and the consequent 

 metabolism of the brain, for cerebral anemia is regularly observed in 

 sleep. If a man be delicately balanced on a tilt-board and then goes to 

 sleep, his head-end rises while his feet-end descends, and this tilting is 

 due to the descent of blood from the head into the trunk and extremities. 

 For the same reason one feels drowsy after a hearty meal, the blood then 

 going to the alimentary canal. 



The pupils of the eyes are contracted during sleep, and their smallness 

 is to some extent an index of the depth of the sleep. This action is 

 apparently due to some central irritation; other sphincters (e.g., of the 

 anus and the bladder) are usefully tightened in a similar way. This 

 tonic contraction is more or less inhibited by afferent sensory impulses. 

 Like the other voluntary muscles, those moving the eyes are relaxed 



