The dependence of the waking state upon the presence and activity of a sufficient quantity of a peculiarly unstable form of protoplasm in the brain is an hypothesis which presents no great difficulty of comprehension. But how may we explain the lapse from the intelligent vivacity of that waking state into the unconscious inactivity of sleep? I have elsewhere[20] discussed the manner in which artificial sleep is produced by impregnation of the brain with anæsthetic substances that interfere with sensibility, and finally produce stupefaction, by hindering the normal processes of intra-molecular oxidation in the protoplasm of the nervous tissues. The same general line of argument may be extended to cover the action of every narcotic agent with which the living substance of the body may become surcharged. Accepting, then, the hypothesis advocated by Obersteiner and Preyer, it becomes an easy thing to account for the gradual onset of sleep by supposing an accumulation of the “fatigue producing” products of intra-molecular oxidation. But we cannot thus explain the rapid and, as it were, voluntary passage from wide awakefulness into a condition of deep sleep, such as may be commonly observed among sailors and others who have formed the habit of going at once to sleep at regularly recurring hours of the day or night. Certain writers have endeavored to account for this fact by imagining a special mechanism at the base of the brain (choroid plexuses of the fourth ventricle, etc.,) by means of which the current of the blood through the brain may be voluntarily diminished, with a consequent arrest of conscious activity. But, still adhering to the hypothesis of Pflüger, we shall obtain a clearer explanation of the facts by considering the phenomena connected with the succession of impressions upon the organs of sense. It has been ascertained[21] that such impressions must persist for a certain measurable length of time in order to excite conscious perception. A sound must be prolonged for at least fourteen-hundredths of a second, a ray of light must agitate the retina for about eighteen to twenty-hundredths of a second, an ordinary contact with the surface of the skin must continue from thirteen to eighteen-hundredths of a second, in order to awaken any knowledge of sound and light and tactile sensation. For the simplest act of perception from two to four-hundredths of a second are necessary. It is, therefore, perfectly reasonable to suppose that when the “explosive material” of the brain has been sufficiently “dampened” by the accumulation of acid refuse which accompanies prolonged cerebral effort, the impressions of sense may no longer suffice to excite in the cortical protoplasm vibrations of sufficient length, or following each other in sufficiently rapid succession, to sustain consciousness. The cortex of the brain may then be likened to a body of water into which bubbles of partially soluble gas are introduced from below. When the bubbles are