Page:Popular Science Monthly Volume 2.djvu/711

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other experiment, made in the following way, furnishes a proof of it. Two dogs were submitted to the section of the nervous centre which contains the vital knot, occasioning death. The appearance of death was produced as soon as the starting-point of the respiratory nerves was deeply injured. By slow degrees the nervous tissues lose their properties, and, before they are entirely extinct, the spinal cords of the two animals are exposed. One is subjected to the action of oxygen gas, and its sensibility increases; the other is influenced by hydrogen gas, and its sensibility remains unchanged. These facts show with absolute certainty that the nervous centres find their conditions of activity in the oxygen of the blood, or, to speak more precisely, the oxygen of the blood-globule.

The brain, the organ of the highest manifestations of life, performs its action like the spinal cord, and an elaborate network of blood-vessels distributes the nutritive fluid throughout all its parts. Yet the mass of the brain does not keep its functional activity constantly at work. The whole organism rests after the day's labor; the brain, when not waking, preserves only its life of nutrition; therefore the religions of ancient Greece, not without reason, regarded Sleep as the brother of Death. The quantity of blood transfused into that organ during these two conditions, so different, of sleep and wakefulness, is not the same. Dr. Pierquin had the opportunity of making observations upon a woman in whom disease had destroyed a large part of the bones of the skull, and deprived the brain of its membranous covering; the nerve-mass, quite exposed, shone with that brilliant lustre observed in all living tissue. While at rest in sleep, the substance of the brain was pink, almost pale; it was depressed, not protruding beyond its bony case. At once, when all the organs were quiet, the patient uttered a few words in a low voice; she was dreaming, and in a few seconds the appearance of the brain completely changed; the nerve-mass was lifted, and prominent externally; the blood-vessels, grown turgid, were doubled in size; the whitish tinge no longer prevails; the eye sees an intensely red surface. The tide of blood increases or lessens in its flow, according to the vividness of the dream. When the whole organism returns to quiet, the lively colors of the infused blood fade away by degrees, and the former paleness of the organ is observed again. The succession of these phenomena permitted the conclusion that increasing action of the cerebral cells attracts a considerable quantity of blood to them.

The general circulation in the brain is weak during sleep; in fainting it suffers complete suppression, and every one must have noticed the effects that result in this organ from the abstraction of blood. The least emotion, the smell of a flower, can bring on impressions that react on the heart, and for a moment suspend its movements; the blood then ceases to stimulate the brain, and paleness of the face indicates the bloodlessness of the deeper parts. The organism no longer puts