supposed loading of the cerebral tissues with the acid products of their own disassimilation during wakeful activity. The acid reaction of the brain and of the nerves after exertion, corresponding with the development of acids in the muscular tissues during contraction, suggested the probability that an excessive presence of lactic acid and its sodic compounds might be the real cause of cerebral torpor and sleep. Could this hypothesis be proved, ordinary sleep would take its place along with the states of unconsciousness induced by anæsthetics and hypnotics, and the lactate of sodium should be found the very best of medicines for the relief of wakefulness. Its administration for this purpose, however, has yielded only the most discordant and unsatisfactory results. The fatigue theory, moreover, is insufficient, since it furnishes no explanation of the invincible stupefaction produced by cold, nor does it render intelligible the unbroken sleep of the unborn child.

Far more comprehensive is the hypothesis advanced by Pflüger.[19] According to this view, the state of wakefulness is maintained by a certain degree of activity in the cortical substance of the brain. Like all other bodily organs, this substance is renovated by the assimilation of nutrient materials derived from the blood. By this process oxygen is stored up in chemical combination, forming “explosive compounds,” whose precise composition is not fully understood. When for any reason the supply of oxygen is insufficient, as in hemorrhage, producing cerebral anæmia, or in impregnation of the red blood corpuscles with carbonic oxide or chloroform, or other substances capable of excluding oxygen from the hemoglobin of the corpuscle, the cerebral tissues are imperfectly renovated. The explosive constituents of the cortical protoplasm are then inadequately renewed after mental activity, and the sensitive portions of the brain are no longer fitted to manifest the highest forms of intelligent activity. But, when nothing interferes with healthy nutrition, the requisite degree of instability in the protoplasm of the brain is effected by intussusception of oxygen. Under the influence of the various nervous impressions which reach the brain, the unstable protoplasmic compounds break up into simpler forms. The motion thus liberated by these “explosions” of excitable matter is, in some way at present utterly inconceivable, projected upon the field of consciousness where the mind dwells; and we are thus brought into conscious relation with the external world.

That the capacity for thus signalling across the gulf which divides matter from mind is the result of a certain perfection and complexity of physical structure is rendered probable by the utter failure of the infra-cortical organs alone to impress the conscious intelligence by any amount of independent activity. The same thing is also indicated by the unconscious sleep of the rudimentary fœtal brain, and by the brevity of the intervals of wakefulness which mark the life of the new born babe. That this capacity is dependent upon a special mobility of the atoms of the brain, is shown by the speedy cessation of intelligence which follows great reduction of temperature, as in hibernation, or during exposure to severe frost. That its exercise is largely dependent upon the activity of the senses is proved by interference with their function, as in the case above quoted (see p. 18) from the observations of Strümpell.

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 large, and when they follow each other rapidly, a continual effervescence is maintained upon the surface of the water. But if the size of the bubbles be reduced, or if the solvent capacity of the liquid be increased, the surface will become almost, if not quite, perfectly tranquil. In some such way, without any great danger of error, may we picture forth the manner in which the generation of ideas in the field of consciousness is related to the molecular movements in the space occupied by the protoplasmic substance of the brain. Returning, now, to the rapid induction of sleep, we find that it is usually the experience of people who lead an active life in the open air, and are compelled to endure frequent interruption of their rest. The sailor who is trained to work four hours on deck, and then to sleep four hours below, has been virtually transformed by this habit into a denizen of a planet where the days