as are possible' within the hard walls of the skull during health are not capable of producing the alternating states of sleep and wakefulness may be concluded from the fact that far more considerable changes take place in disease without the results too hastily attributed to them. The French physiologist, Vulpian, has demonstrated what many English, Continental, and American physicians had long previously conjectured, namely, that the modifications of blood supply and pressure, which have in turn been supposed to be the causes of sleep or wakefulness respectively, are in truth the consequences or concomitants of those states. In short, the changes observed to take place in the rate of the circulation and the size of the vessels of the brain are the results, not the causes of sleep.

For example, under certain circumstances, the suspension of activity in the brain may occur simultaneously with, or induce, a slowing of the flow of blood through the vessels, which seems to indicate stagnation or congestion. More commonly it happens that inaction of the brain and

1 The Causation of Sleep, a Physiological Essay, by James Cappie, M.D., Edinburgh, 1872.

2 Leçons sur l'Appareil Vaso-moteur (Physiologie et Pathologie), faites à la Faculté de Médecine de Paris, par A. Vulpian. Tome second, 144-155.

cerebral nerve-centres tends, as the cessation of functions generally tends, in all parts of the body alike, to diminish the demand for highly oxygenated blood, and, in obedience to the law which makes supply dependent on demand, comparative bloodlessness of the part resting occurs. This is obviously a very different thing from anæmia or bloodlessness as a cause of sleep. It is the simple consequence of the natural law which governs the circulation throughout the body; and probably the condition of anæmia is established in the thought and sense organs, during sleep, as in other parts which are ordinarily the seat of active function; but the blood state is the effect of the sleep state, not its cause.

It

"The blood is the life," or life sustainer. carries the elements of nutrition derived from the food, and oxygen abstracted from the air, to every part of the organism. The circulation is due principally to the pump-like action of the heart, which forces the blood into the arteries at one end of the circuit, or system of vessels, and draws it out of the veins at the other end; but the distribution of blood to particular organs is due to physical and other changes which take place in a part of the circuit which lies midway between the arteries and veins, namely, the capillaries. These vessels form

a network permeating every region of the body, and bringing the blood into due relation with the tissues which require to be nourished. The quantity of the fluid admitted to any organ or tissue depends in large measure on the calibre of the smaller arteries, or arterioles, which bring the blood to the local network, and their size is regulated by the action of a special system of nerves which act upon these vessels, and either cause them to contract so as to admit only a small stream, or relax to allow a larger volume to pass. The nerves fulfilling this important function are directly influenced by the activity of the part, so that when the organ is inactive the supply is reduced. The tissues are endowed with a power of taking up from the blood just what they require, allowing the rest to pass on. If the demands of an organ are large, or the blood flows to it in too small quantity, there will be a reflex nerve action to procure more rapid supply, and the flow will be quickened and the size of the vessels enlarged. When an organ is in morbid action the commotion occasioned is so great that the general pulsation is accelerated, and while what is called "inflammation" occurs in the organ, "fever," or febrile excitement, is set up throughout the body. The brain is in the same position as regards blood supply as every other organ, and

modifications of the circulation through that organ are determined by the common law. When the organs of thought and special sensation sleep the supply is diminished, and the circulation. quickly becomes slow.

2

Some authorities, whose opinion is of great weight-Mr. Durham,1 of London, and Dr. Hammond, of New York, prominently among the number-have conceived that nutrition of the brain goes on chiefly during sleep. They contend, with much plausibility, that when the blood courses rapidly through the vessels it is more likely to be taking up the debris of used tissue than depositing new material; and that when in sleep it passes more slowly through the tissues it gives off the elements required to replace what has been lost, Mr. Durham calls the rapid flow of wakefulness the "circulation of function; "the slower current which passes during sleep he designates the "circulation of nutrition." There is much to be said for this view of the matter, and it is in accordance with the physical law that fluid passes rapidly

1 The Physiology of Sleep. By Arthur L. Durham. Guy's Hospital Reports, 1860, Vol. VI., 149.

2 Wakefulness: with an Introductory Chapter on the Physiology of Sleep; also, Sleep and its Derangements. By William A. Hammond, M.D., Philadelphia.

through the walls of a membranous tube into a rapidly-flowing current; while, on the other hand, the fluid which is in the vessel itself is more likely to transude through its walls when it is flowing slowly than when it passes on rapidly. Against this, however, we must, I think, set the fact that plants do not grow while they sleep during the winter, and, so far as we can judge by analogy, nutrition is more likely to proceed rapidly when an animal organism is in action than when it lies dormant. Those who think the brain is nourished chiefly during sleep seem to me to regard that state as something essentially different from the rest of nature.

The alternations of day and night, of summer and winter, form part of the system of natural life. The animal world has set apart for it, and therefore certainly needs, rhythmical periods of activity and repose. Darkness suggests, and to a people in a state of nature enjoins, the cessation of active exercise. Night is the time for rest. If it were also the period of nutrition-wherein the tissues take up the food-elements from the blood and their cells are nourished-there should be less demand for food on awakening than at any other time; or, if the digestive organs then required a fresh supply in order to elaborate material and enrich the blood for future needs, the indications