1 Kohlschütter, Messungen der Festigkeit des Schlafes, Dissert., Leipzig, 1862, and Zeitschr. f. Rat. Med., 1863.
Sleep, then, is that condition of physiological repose in which the molecular movements of the brain are no longer fully and clearly projected upon the field of consciousness. This condition is normal, and must, consequently, be differentiated from all pathological states which produce its counterfeit by the more or less complete abolition of consciousness. This distinction becomes the more necessary because the presence of the common factor, unconsciousness, in the result has frequently led to the mistake of supposing a community of antecedents in the changes by which the passage from sensibility and consciousness to insensibility and unconsciousness is effected. It needs no argument to show that the insensibility of artificial anæsthesia is a very different thing from the unconscious condition which obtains during natural sleep. Between the stupor of intoxication and the healthful restoration of cerebral energy which accompanies genuine sleep there can be no rational comparison. It is therefore in the phenomena of syncope that the likeness of sleep has usually been sought. The well-known fact that a copious hemorrhage can speedily develop a condition of muscular relaxation, insensibility, and unconsciousness has led certain observers to the belief that natural sleep is, after all, only a species of physiological syncope. The antecedent conditions of syncope are so different from the prodromata of sleep that it is difficult to see how either state can be properly likened to the other. The bloodless condition of the brain which occasions syncope is always the result of pathological or accidental causes, and is not identical with the comparatively insignificant reduction of blood-pressure which may be observed in the brain of the healthful sleeper. It is, moreover, worthy of note that sleep is the normal condition of the unborn child, despite the fact that the growing brain receives a richer supply of oxygenated blood than the trunk and the lower limbs—a state of the circulation quite unlike that of syncope.
Fully impressed by the force of such considerations, certain physiologists2 have reasoned from the analogies suggested by the results of muscular fatigue, and have imagined an hypothesis accounting for the occurrence of sleep by a 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 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 the hypothesis be proved, ordinary sleep would take its place in the anæsthetic category and sodic lactate should be the very best of hypnotic agents. Unfortunately for this doctrine, the administration of the lactates has yielded only the most contradictory and unsatisfactory results. It, moreover, furnishes no explanation of intra-uterine sleep nor of the invincible stupefaction produced by cold.
2 Obersteiner, “Zur Theorie des Schlafes,” Zeitschr. f. Psych., xxix.; Preyer, “Ueber die Ursachen des Schlafes,” Vortrag. Stuttgart, bei Enke, 1877; and Centralbl. f. d. Med. Wiss., 1875, S. 577.
Far more comprehensive is the hypothesis of Pflüger.3 According to this view, the cortical tissues of the brain are recruited by the assimilation of nutrient substances derived from the blood. By this process oxygen is stored up in chemical combination, forming explosive compounds whose precise composition is not fully understood. Under the influence of the various nervous stimulations that reach the brain these unstable compounds break up into simple forms. The motion thus liberated by these explosions is, in some way at present utterly unintelligible to us, projected into 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 separates matter from mind is the result of a certain perfection and complexity of material structure is rendered probable by the utter failure of the infracortical organs of man to impress the conscious intelligence by any amount of independent activity. It 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 the 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 the effects of interference with their function. A recent observation, recorded by Strümpell,4 is very significant in this connection: A young man had lost all power of sensation excepting through the right eye and the left ear. A bandage over the eye and a plug in the ear arrested all communication between his brain and the external world. Thus imprisoned within himself, he gave utterance to an expression of surprise, and sought in vain, by clapping his hands, to arouse the sense of hearing. At the end of two or three minutes these efforts ceased; respiration and pulsation were deepened and retarded, and removal of the bandage exhibited the patient with his eyes closed in genuine sleep.
3 “Theorie des Schlafes,” Arch. f. d. Ges. Physiol., x. 468.
4 “Beobachtungen ueber ausgebreitete Anästhesien und deren Folgen,” Deutsch. Arch. f. klin. Med., xxii.
The dependence of the waking state upon the presence and functional 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 passage from the vivacity of that waking state into the inactivity of sleep? I have elsewhere5 discussed the manner in which artificial sleep is produced by impregnation of the brain with anæsthetic substances, and the same general line of argument may be extended to cover the action of every stupefying agent with which the blood may become overcharged. Could the hypothesis of Obersteiner and Preyer be accepted, it would be an easy matter to explain the advent of sleep when it gradually supervenes; but we cannot thus explain the rapid and intentional passage from wide-awakefulness into the profoundest sleep, such as becomes a matter of habitual experience with sailors and with others who have formed the habit of going at once to sleep at regularly-recurring periods of time. 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 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 find a clearer explanation of the facts by considering the phenomena connected with the succession of impressions upon the organs of sense. It has been ascertained6 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 0.14″, a ray of light must agitate the retina for 0.188″–0.2″, ordinary contact with the surface of the body must continue during 0.13″–0.18″, in order to produce any knowledge of sound and light and common sensation. For the simplest act of perception 0.02″–0.04″ are necessary. It is therefore perfectly reasonable to suppose that when the explosive material of the brain has been sufficiently reduced in quantity and quality by prolonged cerebral effort, the impression of sense may no longer suffice to excite in the cortical substance vibrations of sufficient length, or following each other in sufficiently rapid succession, to sustain the conscious state. The cortex of the brain may then be likened to the surface of a body of water into which bubbles of soluble gas are introduced from below. When the bubbles are large and succeed each other rapidly, the surface is maintained in a state of continual effervescence. But if the size of the bubbles be diminished or if the solvent capacity of the liquid be increased, its surface will become almost, if not quite, perfectly tranquil. In some such way, without any great amount of error, may we picture forth the molecular changes that determine the advent of sleep.
5 Artificial Anæsthesia and Anæsthetics, pp. 15-17.
6 Carpenter's Physiology, 8th ed., p. 852.