the relaxed vessels of the brain the heart injects blood so freely, that the vessels, in like manner as when the frozen hand is held near the fire, become engorged with blood, there is congestion, there is pressure, and there is sleep.
The same series of phenomena from opposite conditions can be induced by narcotic vapors. There is a fluid called chloride of aonyl, which, by inhalation, causes the deepest sleep; during the sleep so induced, the brain is as bloodless as if it were frozen. There is an ether called methylic, which, by inhalation, can be made to produce the deepest sleep; during this sleep the vessels of the brain are engorged with blood.
We are therefore correct in supposing that artificial sleep may be induced both by removal of blood from the brain, and by pressure of blood upon the brain; and in the facts there is, when we consider them, nothing extraordinary. In both conditions, the natural state of the brain is altered; it cannot, under either state, properly receive or transmit motion; so it is quiescent, it sleeps. The experimental proof of this can be performed on any part of the body where there are nerve-fibre and blood-vessel; if I freeze a portion of my skin, by ether-spray, I make it insensible to all impression—I make it sleep; if I place over a portion of skin a cupping-tube, and forcibly induce intense congestion of vessels, by exhausting the air of the tube, I make the part also insensible—I make it sleep.
The two most plausible theories of sleep—the plenum and the vacuum theories I had nearly called them—are, then, based on facts; but still I think them fallacious. The theory that natural sleep depends on pressure of the brain from blood is disproved by the observations that have been made of the brain during sleep, while the mechanism of the circulation through the brain furnishes no thought of this theory as being possibly correct. The theory that sleep is caused by withdrawal of blood from the brain, by contraction of its arterial vessels, is disproved by many considerations. It presupposes that at the time when the cerebro-spinal nervous system is most wearied the organic system is most active; and it assumes that the great volume of blood which circulates through the brain can be cut off without evidence of increased volume of blood and tension of vessel in other parts of the body, a supposition directly negatived by the actual experiment of cutting off the blood from the brain.
There is another potent objection applicable to both theories. When sleep is artificially induced, either by subjecting the brain to pressure of blood, or to exhaustion of blood, the sleep is of such a kind that the sleeper cannot be roused until the influence at work to produce the sleep is removed. But, in natural sleep, the sleeper can always be roused by motion or vibration. We call to a person supposed to be sleeping naturally, or we shake him, and if we cannot rouse him we know there is danger; but how could these simple acts remove 27
