Between Sleep and shadow

The state of general repose which accompanies sleep is of especial value to the organism in allowing the nutrition of the nervous tissue to go on at a greater rate than its destructive metamorphosis. The same effect is, of course, produced upon the other structures of the body; but this is not of so much importance as regards them, for while we are awake they all obtain a not inconsiderable amount of rest. Even those actions which are most continuous, such as respiration and the pulsation of the heart, have distinct periods of suspension.

Thus, after the contraction and dilatation of the auricles and ventricles of the heart, there is an interval during which the organ is at rest. This amounts to one-fourth of the time requisite to make one pulsation and begin another. During six hours of the twenty-four the heart is, therefore, in a state of complete repose. If we divide the respiratory act into three equal parts, one will be occupied in inspiration, one in expiration, and the other by a period of quiescence. During eight hours of the day, therefore, the muscles of respiration and the lungs are inactive. And so with the several glands. Each has its time for rest. And of the voluntary muscles, none, even during our most untiring waking moments, are kept in continued action.

But for the brain there is no rest, except during sleep, and even this condition is, as we all know, only one of comparative quietude in many instances. So long as an individual is awake, there is not a single second of his life during which the brain is altogether inactive; and even while he is deprived by sleep of the power of volition, nearly every other faculty of the mind is capable of being exercised; and several of them, as the imagination and memory, for instance, are sometimes carried to a pitch of exaltation not ordinarily reached by direct and voluntary efforts. If it were not for the fact that all parts of the brain are not in action at the same time, and that thus some slight measure of repose is afforded, it would probably be impossible for the organ to maintain itself in a state of integrity.

During wakefulness therefore the brain is constantly in action, though this action may be of such a character as not always to make us conscious of its performance. A great deal of the power of the brain is expended in the continuance of functional operations necessary to our well-being. During sleep these are altogether arrested or else very materially retarded in force and frequency.

Many instances of what Dr. Carpenter very happily calls "unconscious cerebration" will suggest themselves to the reader. We frequently find suggestions occurring to us suddenly-suggestions which could only have arisen as the result of a train of ideas passing through our minds, but of which we have been unconscious. This function of the brain continues in sleep, but not with so much force as during wakefulness. The movements of the heart, of the inspiratory muscles, and of other organs which perform either dynamic or secretory functions are all rendered less active by sleep; and during this condition the nervous system generally obtains the repose which its ceaseless activity during our periods of wakefulness so imperatively demands. Sleep is thus necessary in order that the body, and especially the brain and nervous system, may be renovated by the formation of new tissue to take the place of that which by use has lost its normal characteristics.

From what has been said it will be seen that the brain is no exception to the law which prevails throughout the whole domain of organic nature-that use causes decay. The following extract from another work[1] bears upon this point, and I think tends to its elucidation.

"During life the fluids and tissues of the body are constantly undergoing change. New matter is deposited, and the old is removed with ceaseless activity. The body may be regarded as a complex machine, in which the law, that force is only generated by decomposition, is fully carried out. Every motion of the body, every pulsation of the heart, every thought which emanates from the encephalon is accompanied by the destruction of a certain amount of tissue. As long as food is supplied in abundance, and the assimilative functions are not disordered, reparation proceeds as rapidly as decay, and life is the result; but should nutrition be arrested, by any cause, for any considerable period, new matter ceases to be formed, and the organs, worn out, act no longer, and death ensues.

"The animal body differs from any inorganic machine in the fact that it possesses the power of self-repair. In the steam-engine, for instance, the fuel which serves for the production of steam, and subsequently for the creation of force, can do nothing toward the repair of the parts which have been worn out by use. Day by day, by constant attrition and other causes, the engine becomes less perfect, and eventually must be put in order by the workman. In the animal body, however, the material which serves for the production of force is the body itself, and the substances which are taken as food are assimilated according to their character by those organs and parts which require them.

"The body is therefore undergoing continued change. The hair of yesterday is not the hair of today; the muscle which extends the arm is not identically the same muscle after as before its action; old material has been removed and new has been deposited to an equal extent; and though the weight and form, the chemical constitution and histological character have been preserved, the identity has been lost."

All this is especially true of the brain. Its substance is consumed by every thought, by every action of the will, by every sound that is heard, by every object that is seen, by every substance that is touched, by every odor that is smelled, by every painful or pleasurable sensation, and so each instant of our lives witnesses the decay of some portion of its mass and the formation of new material to take its place. The necessity for sleep is due to the fact that during our waking moments the formation of the new substance does not go on as rapidly as the decay of the old. The state of comparative repose which attends upon this condition allows the balance to be restored, and hence the feeling of freshness and rejuvenation we experience after a sound and healthy sleep. The more active the mind the greater the necessity for sleep, just as with a steamer, the greater the number of revolutions its engine makes, the more imperative is the demand for fuel.

The power with which this necessity can act is oftentimes very great, and not even the strongest exertion of the will is able to neutralize it. I have frequently seen soldiers sleep on horseback during night marches, and have often slept thus myself. Galen on one occasion walked over two hundred yards while in a sound sleep. He would probably have gone farther but for the fact of his striking his foot against a stone and thus awaking.