Imagens das páginas
[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small]




As we pass nearly the third part of our short lives in sleep it is pleasing to think that the time so spent is not misspent nor lost. Sleep marks that periodical suspension of the functions of animal life, or life of relation, during which the organs that minister to them undergo the restoration of energy which is necessary after a period of activity. Waste of substance, which is the condition and the result of active exercise of function, must be repaired during rest; instead of its being a surprise, therefore, that we sleep, the wonder would be if we did not sleep. In the work and thought of the day is given out by degrees the energy which has been stored up during repose. The need of repair is as true of the organic functions, which never seem to sleep, as it is of the animal functions, which sleep through so large a proportion of our lives. For although an organ like the heart seems not to rest day or night from the first moment of action unto the last moment when it ceases to beat more, yet it plainly rests between each stroke, gaining thereby in alternating snatches of repose the energy for the next stroke; and it is really at rest during a longer period than it is in action-has rested more than it has worked when its life-work is ended. If the heart of an animal which is beating regularly when the chest is opened be made to beat slowly by stimulation of its vagus nerve it will go on beating for a long time; but if its beats are quickened by

irritation of its sympathetic nerve it soon comes to a standstill from exhaustion; nutritive repair and the removal of the waste products of activity cannot keep pace with the rapid consumption of energy in the accelerated pulsations; it is exhausted as the gymnotus is exhausted when it has been provoked to repeated electrical discharges and can give no more shocks until it has recruited its energies. The lowest animal forms, which seem not to sleep at all, probably sleep, like the heart, in similar brief snatches of rest. The organism is a self-feeding and self-repairing machine, but it cannot do its repairs when it is in full work; it must have for its parts, as for its whole, its recurring periods of adequate rest; and the time comes at last when, like any other machine, it wears out, is no more capable of repair, and when the exhaustion which ensues is death-the sleep during which there is no repair and from which there is no awaking.

The conditions under which we go to sleep, the causes which promote it, and the ill effects which follow the deprivation thereof, are proofs of its true purpose in the animal economy. When we wish to sleep we shut out all external stimuli, as a bird puts its head under its wing, banish all subjects of active thought or feeling, and place our bodies in as complete a state of muscular repose as possible: so sleep comes on insensibly as a deeper rest, not as an abrupt change, stealing upon us as darkness upon daylight. The general causes which produce it are such as exhaust the energy of the nervous system, either through suffering or doing, and so occasion fatigue of body and mind; they are muscular and mental exertion, when not too prolonged, the weariness which follows great emotional strain, when not too intense, and severe bodily pain. It is true that we may by a strong voluntary effort, or under the spell of an excitement, prolong the usual period of waking, and resist sleep, although we are very sleepy; but we cannot do so indefinitely, for torpor and incapacity of mental function, delirium, and death are the consequences of an entire deprivation of sleep.

In this connexion it is interesting to ask why we awake— why, once asleep, we do not go on sleeping for ever? Probably very much as the power of the exhausted electric eel to give a shock revives when restoration of energy has taken place by

nutrition during rest. A stimulus to the body, of external or of internal origin, which would have been unfelt during the deep sleep of exhaustion, or would have only been enough to occasion a dream, suffices, as the sleep becomes light through restoration of energy, to awaken the individual either directly or by the vividness of the dream which it occasions. We should not sleep for ever, I believe, if every external stimulus were shut out; for the accumulation of nervous energy would awaken us either spontaneously, or on occasion of the least internal stimulus, which, as the organic functions are not suspended, though they are more languid, during sleep, could not be shut out. If these functions regained their full activity they might directly cause waking. On the time at which we awake habit notably has a great influence within certain limits; when we allow the nervous system so many hours for repose, we accustom it to that allowance, and it learns to do its repairs within the allotted time.

Of what are the physiological accompaniments of the occurrence of sleep we know nothing more than that the circulation of blood through the brain is lowered; not as cause probably, but as coincident effect of the state of nerve-element. Blumenbach long ago took notice in a man whose skull had been trepanned that the brain swelled with blood and rose into the opening when he was awake and thinking, and sank down again when he fell asleep; and the experiments of Mr. Durham, who, having removed circular portions of the skull in different animals, and replaced them by suitable watch-glasses, through which he could observe what happened when the animal was awake and when it was asleep, convinced him that there was considerably less blood in the brain during sleep; its substance then being paler and sinking down, while it reddened and became turgid directly the animal awoke. The fontanelles of young children sink during sleep; and forcible compression of the carotid arteries in the neck of the adult will induce it. There is an active flow of blood to the part where the stimulus of funetional energy attracts and needs it, and when active function is suspended by the recurring necessities of restoring the expended energy by sleep, the circulation of blood falls to the level of

« AnteriorContinuar »