Since the chapter on the Physiology of Sleep was written, I have, by additional experiments, satisfied myself that the theory then enunciated is correct in every essential particular.
By means of an instrument adapted to show the extent of cerebral pressure, and which I first described nearly two years ago, I have been enabled to arrive at very positive results. In every instance the pressure was lessened during sleep and was increased during wakefulness. The experiments were performed upon dogs and rabbits. Briefly, the instrument consists of a brass tube, which is screwed into a round hole made in the skull with a trephine. Both ends of this tube are open, but into the upper is screwed another brass tube, the lower end of which is closed by a piece of very thin sheet india-rubber, and the upper end with a brass cap, into which is fastened a glass tube. This inner arrangement contains colored water, and to the glass tube a scale is affixed.
This second brass tube is screwed into the first, till the thin rubber presses upon the dura mater and the level of the colored water stands at 0, which is in the middle of the scale. Now, when the animal goes to sleep, the liquid falls in the tube, showing that the cerebral pressure has been diminished,—an event which can only take place in consequence of a reduction in the quantity of blood circulating through the brain. As soon as the animal awakes, the liquid rises at once. Nothing can exceed the conclusiveness of experiments of this character. No mere theorizing can avail against them.
Footnotes:
[1] See the author’s Treatise on Hygiene, page 92.
[2] La Théorie des Songes. Paris, 1766, p. 206.
[3] On Obscure Diseases of the Brain, etc. London, 1860, p. 604, note.
[4] Physiologie de la Pensée. Recherche Critique des Rapports du Corps à l’Esprit. Deuxième édition. Paris, 1862, t. ii. p. 440.
[5] Du Sommeil, des Rêves et du Somnambulisme, etc. Lyon, 1857, p. 14.