THEORIES OF SLEEP. 433 



Pfliiger's theory has little experimental evidence in its favor. We 

 know that a bloodless muscle may be subjected to a vacuum and made 

 to part with its free oxygen, but that it is still capable of doing much 

 work and of giving off carbon dioxide. In other words, oxidation 

 may take place in the absence of free oxj'gen. Of course there is but 

 one reasonable explanation, namely, that there is a store of the ele- 

 ment in loose chemical combination. This store in the cells is spoken 

 of as ' intra-molecular ' oxygen, and its amount may be supposed to 

 vary between rather wide limits. Pfliiger pointed out that during the 

 day, the katabolic processes being above the average, this hoard might 

 be reduced until the lack of it should lead to the depression of func- 

 tional activity and the suspension of consciousness characteristic of 

 sleep. 



Perhaps no one will maintain that this theory is adequate by itself. 

 If there were nothing but intra-molecular oxygen to be considered, 

 we should expect that a day of idleness would leave one fresh and 

 bright at bed-time and that severe exercise for half a day might make 

 a long sleep a pressing necessity. Pfliiger's idea seems to explain 

 more readil}^ the sensation of being tired than that of being sleepy, 

 which is so often quite independent of the other. 



The alternative theory is to the effect that the waste-products of 

 metabolism are not fully and promptly removed as they are formed 

 during the day's activities, but gradually clog and poison the system 

 until torpor is induced. The lactic acid produced in muscular con- 

 tractions is held responsible for a great part of this toxic process. 

 Acidity of the blood produces coma, and whatever reduces its normal 

 alkalinity might be expected to favor sleep. Many objections to this 

 theory suggest themselves. It does not explain why many people are 

 at their best late in the day, nor why the onset of sleep is relatively 

 sudden, nor why we are sleepy in the height of digestion when the 

 blood is most alkaline. It is perhaps less easy to assail it if we sup- 

 pose that the waste-products in question are not at large in the blood 

 but have accumulated in certain cells, especially of the nervous system. 

 In this case we need not assume a large quantity of these narcotic 

 poisons, but only a peculiar distribution, and we can see why mental 

 work is quite as fatiguing as physical work. 



The transition from wakefulness to sleep seems rather abrupt, but 

 is not instantaneous. Motor control is generally lost before sensation, 

 and most people agree that of the avenues of communication with the 

 world without, hearing is the last to be closed. This order of events 

 is reversed in waking, when the alarm-clock or the unwelcome call is 

 heard for an appreciable time before the eyes can be opened or a 

 definite sense of one's situation realized. The sinking into sleep is 



VOL. LXIII. — 28. 



