for food would be purely visceral.

we know, the fact.

This is not,

When the blood is deficient in the elements of nutrition the brain betrays instant consciousness of its poverty. This was admirably shown by that distinguished and practical observer, the late Marshall Hall, in his elucidation of what he designated "The Temper Disease," a slight but significant variety of which malady occurs when man or any animal is kept waiting for his meal beyond its accustomed period. The irritation thus produced is generally allayed with such celerity as forbids the belief that there is usually a long interval between taking a supply of food into the stomach and brain nourishment.

Again, if sleep were the time for brain-feeding, drowsiness would not take place until after the lapse of an interval sufficient to allow of digestion, because nutrition cannot begin until that process is, at least, nearly complete; some of the most important elements of the food being the last to become soluble. Food must be elaborated in the organs of digestion before it can enrich the blood, whereas we know the tendency to sleep which follows upon a meal is strongest immediately, or very soon, after eating. In short, food acts too rapidly as a restorative to the brain and sense organs during wakefulness to support the theory that sleep is the

period of nutrition. Moreover, it is not uncommonly found that the brain cannot rest if the stomach be overloaded, or the blood charged with too rich or abundant food-material; while a nap before a heavy meal refreshes, and invigorates and promotes brain and nerve nourishment. If, as some have supposed, the cerebrum and sense and nerve centres slept when exhausted, laid in a new stock of potential energy during sleep, and awakened when the reserve was complete,1 heavy brain-work carried to the verge of exhaustion should conduce to sleep, and the recuperated mind-organ would awake in high activity; neither of which assumptions are supported by experience. The over-wearied brain-worker is commonly unable to sleep, and few men awake with their intellectual energies in the fullest state of efficiency, or are fit for the business of the day until they have fed. It is, on the whole, probable that the brain, like other parts of the organism, requires rest as well as nutrition, and that the two needs are not satisfied by the same state. Sleep provides the repose, nutrition goes on side by side with work. I cannot, of course, affirm that the conclusion at which

A theory propounded by Sommer, based on the experiments of Pettenkofer and Voit, as to the comparative consumption of oxygen during quiescence and in exercise.

I have arrived is the true solution of this interesting problem; and it is so much at variance with the view generally taken that, after years of special research on the subject, I can only state it with deference to those who speak with higher authority, on a subject of physiology, than any psychologist can claim. Meanwhile, looking at the matter from a practical standpoint, it is certainly convenient to consider sleep as a state wherein the brain rests, which, indeed, is the fact, whether it is required to feed while it rests, or should, as I believe, remain in a condition as nearly as possible one of inaction.

Ex

What, then, is sleep? To this I answer, in its full development, it is a state of physiological rest. There are many grades of sleep. perimenters have ascertained that the intensity or depth of sleep goes on increasing for about an hour after its commencement; then follows a stage of fluctuating depth, in which the sleeper may be more readily disturbed; and some time before the usual hour of awakening the return to consciousness begins, proceeding gradually by successive lightenings of the condition until at last the brain wakes spontaneously, or is easily aroused. Dreams are the results of defective or partial sleep, and their common occurrence in the lighter

varieties of the state shows that the rest taken by most persons is not either profound or continuous, even while it lasts. The purpose of sleep is probably to take the strain off the consciousness and give it, so to say, new points of departure. In the nearest approach to perfectly normal sleep the sensibilities are dulled to external impressions, cerebration is, I believe, suspended, and the mind ceases to act. During such a state it may well be that the germs of animal life lie dormant like the vital principle in the vegetable organism, or in the system of the hybernating animal. That vital

changes may take place in the tissues during sleep is probable, just as chemical forces may work in the dead body; but the analogy of the seed which retains its vitality during a long period of inaction without any interchange of constituent elements with the material of the outer world, seems more pertinent to the comprehension of sleep.

It is important to recognise this point, as it has a practical bearing upon the estimate we should form of sleep, the sort of sleep with which we ought to be satisfied, the steps that should be taken to prepare for it, and the mistakes against which it is necessary to guard in attempting to procure or prolong it. If sleep is to be a state of rest it should be so conditioned by the

a blow on the head, or seems to be produced by "compressing the vessels of the neck," the immediate condition set up is morbid. The sleep that sometimes occurs later on in this experiment is reactionary, and intended by nature to be restorative. Again, if the brain and cerebral centres are deprived of their blood-supply by failure of the heart's action, as in faintness, or by loss of blood from any cause, there will be yawning and "sleepiness; " but the state induced is not sleep. It is one of exhaustion tending towards syncope and death.

If the blood be poisoned with carbonic acid, or if it be insufficiently supplied with oxygen-for example, from repeatedly breathing the same atmosphere in a close or crowded room, or from some defect in the number or efficiency of the oxygen-carriers, the red corpuscles of the blood, as when these are affected, perhaps shrivelled and contracted, by the presence of certain chemical properties in the blood, or during the administration of chloroform, or after taking chloral-insensibility may supervene, but not sleep; and if sleep follows, it is easy to recognise the difference of the two states. That such changes in the blood pressure

1 Dr. Fleming, in a Paper in the British and Foreign Med. Chirurg. Review, 1855, Vol. I., p. 529.