Pflüger'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 oxygen. Of course there is but one reasonable explanation, namely, that there is a store of the element 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. Pflüger 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 functional 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. Pflüger's idea seems to explain more readily 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 contractions 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 suppose 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
