the separation of a liquid, to the formation of which the blood must more or less give its aid; it rather expresses a phenomenon generally remarked in all the organs, whatever the nature of their function may be. The muscular system, which produces nothing but mechanical work, is in this regard like the glands, which act chemically. At the instant of muscular action the blood circulates with greater activity, which relaxes when the organ begins to rest. The peripheral nervous system, the spinal marrow, and the brain, which serve to manifest the phenomena of innervation and intelligence, are equally subject to this law, as we are about to see.
The relations existing between the phenomena of circulation in the brain and the functional activity of that organ have long remained obscure, owing to mistaken ideas of the conditions of sleep, which is rightly considered the state of rest of the cerebral organ. The ancients suppose that sleep resulted from compression exerted on the brain by the blood when its circulation declined. They imagined that this pressure was chiefly exerted at the back part of the head, at the point where the veined folds of the dura mater unite in a common confluent, which is still called the torcular or compress of Herophilus, from the name of the anatomist who first described it. These conjectural explanations have been handed down to us; and it is only of late years that experiment has succeeded in proving their falsity. In fact, it has been shown by direct experiment that, during sleep, the brain, instead of being congested, is on the contrary pale and bloodless; while in a state of wakefulness the circulation, becoming more active, provokes a flow of blood proportioned to the intensity of cerebral activity. In this respect natural sleep and the anaesthetic sleep of chloroform are alike; in both cases, the brain, sunk into rest or inactivity, presents the same paleness and relative bloodlessness.
The experiment is made in this manner: A part of the bony covering of an animal's skull is carefully removed, and the brain laid bare so as to study the circulation at the surface of this organ. Then chloroform is administered to produce insensibility. In the first exciting stage of the action of the chloroform, the brain is observed to grow congested and to lap over at the edges; but as soon as the stage of anaesthetic sleep is reached, the substance of the brain sinks in and grows paler, presenting a languid movement of capillary circulation, which lasts as long as the state of sleep or cerebral rest continues. For the study of the brain in natural sleep a circular trepan is made on a dog's head, and the piece of bone removed is replaced by a watch-glass carefully adjusted to the exact opening, so as to prevent the irritating action of the air. The animals subjected to the operation survive it; and observations on their brain through this sort of window, while awake and when asleep, prove that when the dog is asleep the brain is always paler, and that a fresh afflux of blood is regularly noticed on his awaking, when the functions of the brain resume their activity.
