THE NATURE AND CAUSE OF SLEEP.

Natural sleep 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 universally observed in all healthy animals; and its recurrence is intimately associated with the diurnal revolution of the earth, and the succession of day and night. The disappearance of daylight is, for the majority of living creatures, the signal for cessation of active life. Though its onset may be for a time delayed by an effort of the will, the need of rest at length overcomes all opposition, and the most untoward circumstances cannot then prevent the access of unconsciousness. The story of the sailor-boy, sleeping on “the high and giddy mast,” is familiar to every one. An officer in the United States Navy has assured me of more than one instance in which men had fallen asleep under his own eyes, oppressed by exhaustion, during the roar of a long continued bombardment. Thus produced, the relation of cause and effect between weariness and sleep becomes very apparent. The refreshing influence of such repose points clearly to the restorative character of the physiological processes which persist during the suspension of consciousness. It also renders evident the final cause of that periodical interruption of activity which the brain experiences in common with every other living structure.

Sleep is usually preceded for some time by a feeling of sleepiness. This sensation, like the analogous sensations of hunger and thirst, represents in some measure the progressive diminution of energy throughout the entire body; but it is chiefly expressive of the failure of cerebral energy. It produces a sense of general heaviness and intellectual dullness; the special senses become less alert, the eyelids droop, numerous groups of muscles experience the spasmodic contraction of yawning, the head drops forward and is recovered with a jerk, the limbs relax, and the whole body tends to assume a position convenient for repose. Every school-boy who has been compelled to pass an evening hour at a dull lecture, under the eye of a martinet monitor, will testify to the suffering which attends any unusual prolongation of this period. But, if the natural course of events be not obstructed, the stage of mere sleepiness is soon passed, and the introductory stage of sleep is entered. This is a state in which the individual is neither awake nor fully asleep. It is known as the hypnagogic state. During this period the phenomena of simple sleepiness become exaggerated to such a degree that the attitude of repose is assumed without effort if the body be permitted to follow the natural inclination of its different members. The eyes close, the other senses become inactive, though the sense of hearing is the most persistent. Released in considerable measure from the control of the brain, the reflex energy of the spinal cord is at first somewhat exalted. Witness the fibrillary twitching of the muscles, and the convulsive state, which may often be observed during the stage of somnolence after severe fatigue. The uneasy sleeper may even be roused to complete wakefulness by such involuntary movements. But, as sleep becomes more profound, the reflex functions of the cord are also weakened.[1] As the sensory organs retire from action, the intellectual faculties lose their equilibrium. First, the power of volition ceases. Then the logical association of ideas comes to an end. The reasoning faculty disappears, and judgment is suspended. We become, therefore, no longer capable of surprise or astonishment at the vagaries of memory and of imagination, the only faculties that remain in action. To their more or less unfettered activity we owe the presentation in consciousness of those disorderly pictures which, occurring in this stage of imperfect sleep, have been termed hypnagogic hallucinations.[2] During the early moments of this period an observant person may often retain a power of reasoning sufficient to remark the fact of dreaming, and this effort of attention may produce a partial awakening; but, usually, the subsidence of cerebral function is progressive and rapid. The fire of imagination fades, the field of consciousness becomes less and less vividly illuminated, the entire nervous apparatus yields to the advancing tide, and, finally, the dominion of sleep is fully confirmed. The sleeper knows nothing of the external world, and has lost all consciousness of his own existence. But the duration of profound repose is brief. From the end of the first hour the depth of sleep, at first, rapidly, then, more gradually, subsides. Dreams disturb its tranquility, mental activity increases, the power of volition revives, and, at the end of six or eight hours, the individual is once more awake. The subjoined diagram, borrowed from the Dictionaire EncyclopÉdique des Sciences MÉdicales, will facilitate the apprehension of these successive phases in the course of sleep:

It was formerly believed that during the time of sleep all the processes of assimilation and nutrition throughout the body are increased,—in short, that it is the season of repair for the waste of tissue incurred during the hours of wakeful activity. While it is true that in sleep the expenditure of force is greatly reduced, the more exact researches of modern physiologists indicate a universal reduction in the rate of all the vital processes. The final result, however, is a general renewal of energy, because the aggregate income of the tissues is greater than their outgo during the suspension of conscious activity. The following observations make very apparent the fact of a reduction of physiological activity:

Respiration.—The process of breathing is conducted with greater deliberation during the period of sleep. This reduction is one of the most notable of the circumstances that first attract the attention of the spectator who observes a sleeping person. The average number of respirations per minute, in an adult of twenty-five to thirty years of age, is sixteen. Quetelet remarked[3] that during sleep this number was diminished by about one-fourth. The same fact has been recorded by other observers.[4] Mosso has also noted the fact[5] that there is a change in the type of respiration, the movements during sleep become less diaphragmatic and more largely costal. He furthermore observed that during the waking period the act of inspiration consumed 8-12 of the complete respiratory phase, but during sleep it was prolonged till it occupied 10-12 of the same cycle. The interval between the end of expiration and the commencement of inspiration was also obliterated by sleep. Notwithstanding this relative increase of inspiratory motion, the quantity of air that passes through the lungs is considerably reduced by reason of the diminished action of the diaphragm. A corresponding reduction of the gaseous exchanges between the blood and the external air has been determined by the experiments of Pettenkofer and Voit, Boussingault, Lewin, and other equally competent observers.[6]

Circulation.—During sleep the heart beats less frequently than during the waking hours. Though a portion of this delay must be attributed to the recumbent position, sleep does still further retard the movement of the heart. My own observations upon children in bed exhibit a difference of twelve to sixteen beats between the pulsations when awake and asleep. According to Trousseau[7] the average number of pulsations observed in a group of thirty children, varying in age from fifteen days to six months, was 140 when awake and 121 when asleep. In another group of twenty-nine children, between the ages of six months and twenty-one months, the average was 128 when awake and 112 when asleep. The observations of Hohl and Allix[8] indicate that among very young children the difference between the pulse of sleep and the pulse of wakefulness may equal forty beats. According to Guy (loc. cit.) the pulse is more variable in the morning than during the afternoon or evening.

Temperature.—Aside from the almost hourly fluctuations of the bodily temperature, a noticeable sinking of the temperature-curve is observed during the hours of sleep. This alone is sufficient to indicate a diminished rate of combustion in the tissues. Boussingault found[9] that a dove which consumed 255 millegrammes of carbon every hour while awake, oxidized only 162 millegrammes when asleep. Scharling also observed that the quantities of carbon successively oxidized by the same man when asleep and awake bore to each other the ratio of 1:1.237. The observations of Demme[10] indicate that increase of bodily temperature during the hours of sleep must be considered as the result of pathological processes in the tissues. The observations of Allix (loc. cit.), made upon sixteen children during the first twelve days after birth, showed an average fall of 0°.38 C. during the hours of sleep. Eight children, between five and sixteen months old, exhibited a similar depression of 0°.56 C.; while ten children, ranging in age from twenty months to four years of age, averaged 0°.34 C. less when asleep than when awake.

The well-known experiments of Chossat, who found that the temperature of pigeons was from 0°.70 C. to 0°.90 C. higher at noon than at midnight, may not be considered satisfactory evidence of the depressing influence of sleep, because it is true that the diurnal variations of temperature which are conditioned by the vital activities of every animal might be sufficient to account for these differences. The experiments of Horvath[11] are more convincing. This observer found that the marmots upon which he experimented were accustomed to sleep during the winter for about four days continuously, and would then remain awake for an equal length of time. “During the sleeping period they can be cooled down to such a degree that a thermometer introduced into the rectum to the depth of an inch and a half indicated only 3°F. above the freezing point. The temperature rose rapidly after the animal awoke, so that in the course of an hour it was 3°F. higher; at the close of the second hour 9°F. higher, and at the end of the next half hour about 27°F. * * Neither respiration nor the muscular movements were correspondingly augmented.” This observation clearly shows the powerful influence of cerebral activity upon the liberation of heat within the body.

Secretion.—The functions of the numerous glands throughout the body are diminished during sleep. The tears dry up, and the cornea receives less moisture. Hence the stickiness of the margins of the eyelids during the sleep of a patient suffering with conjunctivitis. He can open his eyes, on awaking, only after sufficient time has elapsed to revive the lachrymal flow. Exner[12] remarks the diminution of pathological secretion in nasal catarrh during the hours of sleep. The mouth in like manner ceases to receive its full quota of saliva, and its cavity quickly dries if the lips remain open. The secretions of the gastro-intestinal glands vary with the contents of the alimentary canal; but in general they are considerably diminished, and digestion is correspondingly retarded during the hours of sleep. The quantity of urine is lessened during sleep.[13] The elimination of urea and of other excrementitious matters is less during the night than by day.[14] Unless increased by disease, or by accidental circumstances connected with atmospheric temperature and unnecessary clothing, the perspiration is also diminished.

Nutrition.—All the molecular processes of nutrition are reduced by sleep. The lowering of the bodily temperature has been already indicated. The observations of Helmholtz[15] indicate that the actual liberation of heat in the tissues is but little more than one-third of the amount set free in an equal period of time during the waking hours. The numerous experiments[16] of Boussingault, Henneberg, Scharling, E. Smith, Liebermeister, Pettenkofer, Voit, and Lewin, clearly indicate the fact that during sleep less oxygen is absorbed, and less carbonic acid gas is discharged, by the tissues. Voit found that while, during the daytime, 435 grammes of oxygen were taken in by a working man, only 326 grammes were needed by the same individual during the nocturnal half of the day. Artificial sleep occasioned by chloral hydrate produced a similar reduction in the consumption of oxygen and in the formation of carbonic acid gas. Under the influence of morphine the reduction of CO_2. reached 27 per cent., and the diminution of oxygen amounted to 34 per cent. of the quantities furnished during wakeful activity. The comparatively small reduction (only 6 per cent.) in the decomposition of the nitrogenous elements of the body during the same period, exhibits the close relation between the metamorphosis of the non-nitrogenous elements of the tissues and the amount of bodily activity.

The experiments of Pettenkofer and Voit, to which allusion has just been made, serve also to illustrate the fact that all tissue changes are increased by every excitement of the sensory organs of the body, but are diminished by the subsidence of peripheral irritations. Hence the importance of quiet and darkness when we seek to induce that state of the body in which molecular processes should reach their minimum. Since every act of perception is attended by an outburst of refuse matter from the nervous tissue, the quantity of such excrementitious discharge in any given period of time becomes in some sort a measure of the vital activity of the organism. Conclusive proof of the diminution of vital function during sleep is thus obtained.

It must not, however, be inferred that the general reduction of tissue-change, which has thus been established, during the hours of sleep, is evidence of a universal and uniform reduction of function throughout the body. Sleep seldom falls at once with equal force upon every organ; its invasion is progressive. Consequently, certain structures may be fast asleep, while others are partly awake,—while still other portions of the organism may be in a condition of activity greatly in excess of their ordinary wakeful function. Upon this fact depend the phenomena of dreams and the various forms of somnambulism. The special senses are usually overcome by sleep before the muscular apparatus yields, and the cerebro-spinal nervous centres are the last of all to succumb. The eyes, for example, cease to see clearly before the eyelids droop, or the muscles of the neck give way in the act of nodding. The senses of touch and of taste fail next in order, as in the case of the infant gourmand, who may be seen falling asleep at supper,—his mouth yet filled with untasted sweets from the table before him. The sense of smell is more persistent, and its exercise is sometimes an obstacle to the invasion of sleep. Witness the effect of powerful odors upon certain persons. The perfume of flowering plants in the sleeping chamber is sometimes decidedly annoying on this account. A lady of my acquaintance was once awakened out of a sound sleep by the smell of tobacco smoke from the pipe of a thoughtless burglar who had quietly entered a distant apartment of the house. A sudden change of wind, deluging a city with the vapors of a glue-factory or rendering establishment, may in like manner disturb the slumbers of thousands of people.

The sense of hearing seems to be the most persistent of all the special senses. It is not a very uncommon thing for persons to be awakened by the sound of their own snoring; or, if not actually aroused by the noise, to remain in a condition of repose which seems to be sustained and cheered by the regular rhythm of its own music. As a general rule, however, it is noteworthy that, when not wholly dormant, each sense finds its sphere of activity greatly narrowed by the fact of sleep. Consequently the range of perception, if not wholly obliterated, is greatly limited during the time of sleep.

While it is true that sleep arrests the voluntary activities of the muscles, it is also a fact that all the muscles do not yield at once or in equal degree. The extensors of the neck, and the supporters of the spinal column, are the first to fail. The patient begins to nod, and is inclined to fall forward, before consciousness ceases. The muscles of respiration and of circulation continue to contract, though at a diminished rate. The vermicular movements of the intestinal coats persist, and in certain conditions of ill-health their exaggerated contractions may become a cause of imperfect repose. Reflex movements may always be excited during natural sleep. Tickling the sole of the foot will cause retraction of the limb; and before the complete establishment of sleep, a certain exaltation of the spinal reflexes may be observed. Young children may frequently be seen in the act of suction with their lips, as if at the breast; and the smile of the sleeping infant is a matter of daily remark in every nursery. The influence of dreams as an excitant of muscular movement will be hereafter discussed.

The variation of intellectual function which appears in sleep serves to measure its profundity and to indicate the extent of its invasion. The act of perception being dependent upon sensation, it is to be observed that the range of perception diminishes so soon as the organs of sense begin to yield. Its intensity may not immediately fail, but the breadth of its scope is narrowed. Sometimes, however, the act of conscious perception is arrested before the organs of sense are sealed. The sleepy reader may continue to eye the page before him, perhaps even to read aloud for a considerable time after he has ceased to derive any meaning from the words of the book. In such cases the organs of perception and conception and association of ideas slumber before the bonds of connection between the will and the muscular organs have been completely relaxed. Such an example affords a valuable illustration of the division of the brain into separate mechanisms which, though most intimately related, are nevertheless partially independent of each other. Sleep may operate like an invasive disease, falling with unequal incidence upon the different structures that make up the mass of the brain, paralyzing one portion, while simply benumbing another, and even arousing to excessive activity a third. Consequently the intellectual functions may be very unequally disturbed, and the order of their subsidence may be considerably varied; but, as a general rule, the physiological relations of the faculties are respected, so that as sensation diminishes, perception fails, the conception of ideas is correspondingly hindered, and the association of such ideas as are still projected upon the field of consciousness becomes more imperfect. The loss of the power of association implies the destruction of memory and the impossibility of exercising the reasoning faculty or of forming those judgments upon which every act of volition is based. When the brain has at length been so far overwhelmed that physical impressions can no longer reach the field of consciousness, all manifestation of intellectual life is at an end, and the sleeper sleeps a dreamless sleep that leaves no trace behind.

It is assumed in the last sentence that the brain may become so far transformed by sleep that it ceases for the time to be capable of function as the instrument of thought. This conclusion has been questioned by the very highest authorities. Sir William Hamilton, Exner, and many others have instituted numerous experiments to test the possibility of a dreamless sleep. Causing themselves to be suddenly aroused at all hours of the night, they invariably found themselves at the instant of awaking occupied with the course of a dream. Hence it has been inferred that the mind is always alert, even when the body is most thoroughly asleep. In explanation of the fact that consciousness contains after deep sleep no trace of such mental activity, it is claimed that the act of dreaming of which we are aware at the moment of waking is proof of intellectual function during the moments which preceded that incident, and that we are merely forgetful of all similar processes that occurred during undisturbed sleep. The unconsciousness of sleep, according to this theory, is not real—it is only apparent through failure of the memory. If this be true, memory is the only intellectual faculty of whose inaction we can be sure. The period of deep sleep might then be, for all we know to the contrary, a period of the most intense and exalted mental activity. But, if so, it is quite worthless as a constituent of our conscious existence. It may also be objected with equal reason that the dreams which unquestionably occupy the field of consciousness at the instant of waking are probably excited by the impressions which terminate sleep. The process of waking, though often very greatly hurried, is by no means absolutely instantaneous. As we shall learn, the time requisite for the evolution of a dream may be indefinitely brief. Consequently, it seems better in all such instances to assign the period of dreaming to the time of diminishing slumber that corresponds to the disturbance by which sleep was terminated.

The only reason for any hesitation in the acceptance of such a proposition consists in the reluctance of many philosophers to admit the possibility of any interruption in the active life of a spiritual being, such as man is conceived to be. But it is difficult to comprehend any valid reason for the denial of such interruption. Every form of force, of which we have any knowledge, is subject to fluctuations in the course of its phenomenal manifestation. When a physical force ceases to exhibit itself in an active state, and passes into a potential modification, we are not compelled to regard it as extinguished. It is merely latent or inhibited, but always ready to take its place again among the kinetic forces of nature. In like manner there seems to be no good reason why that spiritual force or congeries of forces which constitutes the mind of man may not experience analogous transformations in successive periods of action and of repose. Such periods of rest occur in sleep, in coma, in disease and disorganization of the brain. The mind sleeps, it does not cease to exist—probably not even when death dissolves its material substratum.

That the depth of sleep is exceedingly variable is evident in the experience of every one. A German physiologist[17] has made a rough estimate of the soundness of sleep by comparing the loudness of the noises necessary to wake the subject of experiment at regular intervals during the course of the night. He arranged a gong with a pendulum attachment, and noted the length of the stroke which produced a sound sufficiently loud to awaken the patient. In this way the different degrees of intensity of the awakening noise could be calculated, and the corresponding depth of sleep could be estimated. It was thus concluded that the depth of sleep increases rapidly during the first hour, at the end of which time it has reached its maximum. During the next half hour it diminishes as rapidly as it had increased in the first half hour. During the next hour it still further diminishes, almost as much as it increased during the second half hour. The remaining ten half hours of the experiment were occupied by a comparatively light and gradually diminishing slumber, until the vanishing point of sleep was reached at the expiration of eight hours from its commencement. This observation corresponds with the general opinion that sleep is deepest in the early part of the night. For the same reason dreams and wakefulness are most frequent during the early watches of the morning.

When considering the causes of sleep it is needful to exclude from view those artificial varieties of sleep that are produced by the various narcotic drugs, as well as the counterfeits of sleep which result from diseased conditions of the body. It is comparatively easy to frame hypotheses in explanation of such interruptions of our conscious life; but, when we attempt to formulate a theory which shall satisfactorily account for the occurrence of natural sleep in healthy animals, the task becomes exceedingly difficult.

First among the causes of sleep may be reckoned the alternation of day and night. With the disappearance of sunlight all nature sinks into a condition of repose.

“The night brings sleep
To the greenwoods deep,
To the bird of the woods its nest;
To care soft hours,
To life new powers,
To the sick and the weary—rest!”

In this tendency to nightly inaction man shares with all other living creatures. His body thus testifies to the intimacy of its relations with all portions of the solar system. Originated in the tropical regions of the earth, where day and night are nearly equal, we find in all parts of the world the same hereditary need of a period of rest, nearly coincident with the duration of the shorter nights of the tropical year. Had the birth-place of primeval man been situated within the Arctic circle, it is probable that his hours of sleep might have differed considerably from the number now needed by the average individual. So powerful are the necessities thus dependent upon the harmony between our organization and the movements of the earth, that if the habit be formed of sleeping at other hours than those which are usually devoted to that purpose, the full complement of sleep is still needful to satisfy the demand for rest.

Prominent among the causes which predispose to sleep at night is the cessation of a majority of the sensations that are continually pouring in upon the brain during the period of daylight. Hence the necessity for seclusion in darkened rooms, from which the noises of the daytime are shut out, if one would sleep during the long days of the arctic summer, or if one would enjoy a midday nap at any season of the year. The close dependence of wakefulness upon the constant activity of the organs of sensation, is well illustrated by a case related in Hermann’s Handbuch der Physiologie, Vol. II, Part 2, p. 295. A young man had been reduced by disease to such a condition of general anÆsthesia that the right eye and the left ear were the only remaining paths of sensation between his brain and the external world. Whenever the sound eye and ear were bandaged so as to cut off all communication with the brain, the patient invariably fell asleep in the course of two or three minutes after the interruption of sensation. In like manner, some people, even in perfect health, are able to sleep at any time by simply lying down and closing the eyes. Such persons, however, are not often very highly gifted in the intellectual sphere. They generally belong to a class of men whose lives are laborious and liable to great irregularity and fatigue. Such people labor in the open air, where every organ of sense is in a state of continual excitement. As soon, therefore, as they can find a quiet corner from which the commotion of the elements is excluded, it is only necessary to close the eyes—the principal avenue of communication with the outside world—and sleep begins at once. This is especially true if severe bodily exertion has preceded the opportunity for repose.

Fatigue of any sort is one of the most energetic causes of sleep. The impossibility of long sustained exertion is a fact almost too familiar to attract attention. Every muscle must be suffered to rest for a time after contraction before it can be again contracted. Even the heart and the muscles of respiration must be allowed to enjoy regular periods of repose many times each minute. These are examples of local rest, not involving the entire body. But if the whole body participate in any violent action, every part will manifest a consequent disposition to rest. Witness the effects of the venereal act. Every muscle is relaxed; the brain, which has officiated as the supreme source of energy, experiences exhaustion, and sleep frequently terminates the voluptuous paroxysm. In like manner, sensations of severe pain, if sufficiently prolonged, become a cause of sleep. Prisoners upon the rack have slept through sheer exhaustion while undergoing the horrors of torture. Little children frequently fall into a deep sleep immediately after painful, though comparatively bloodless, surgical operations performed without anÆsthetics. The depressing emotions, even, may so fatigue the brain as to induce sound sleep through reaction from previous excitement. Every wearied portion of the body must rest; and when the brain thus rests, sleep is the consequence.

Impressed by the force of such considerations, certain physiologists[18] have reasoned from the analogies suggested by a study of the results of muscular fatigue, and have suggested 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, 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 foetal 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 and the nights are each but four hours long. His bodily functions become accommodated to this condition; his nervous organs store up in sleep a supply of oxygenated protoplasm sufficient only for an active period of four or five hours; so that, when the watch on deck is ended, he is in a state as well qualified for sleep as a laborer on shore at the close of a day of twelve or fifteen hours. Moreover, the majority of those who can thus easily fall asleep are individuals whose waking life is almost entirely sustained by external impressions. So soon, therefore, as such excitants are shut out by closing the eyes in a place of shelter from the sounds and turmoil of the air, comparatively little remains for the stimulation of ordinary consciousness, and sleep readily supervenes through mere lack of cerebral excitement, especially if the excitable matter of the brain has been previously overwhelmed by the products of active exertion.

That analogous, though not identical, predisposition to unconsciousness may also be rapidly induced by modifications of the cerebral circulation is proved by the sudden reduction of cerebral excitability and consciousness which occurs during the act of fainting. In this counterfeit presentment of sleep the important part played by variations of the blood current through the brain is so conspicuous that certain writers have attempted to show that genuine sleep is the result of a diminution in the flow of blood to the cortex of the brain. An ingenious physician has even attempted to relieve insomnia very much as a surgeon might undertake to cure a popliteal aneurism—by placing tourniquets on the arteries leading to the affected part. But the mere fact that syncope produces unconsciousness does not prove that “cerebral anÆmia” should be elevated to the rank of the principal cause of natural sleep. The nervous process is the primary factor. The circulation of the blood is everywhere under the immediate control of the nervous system. Consequently, every change in the condition of the nervous structures is followed by a corresponding change in the state of the circulating apparatus. Wherever an organ is aroused to activity, so delicate are the adjustments by which it is connected with the brain and with the heart that it is at once irrigated by an increased flow of blood. When its functional activity subsides, the same mechanism provides for a corresponding reduction in the supply of blood to its tissues. The brain itself forms no exception to this law. This has been admirably shown by the observations of Professor Mosso, of Turin.[22] The learned professor enjoyed the rare opportunity of observing three individuals who had suffered the loss of a considerable portion of the bony walls of the cranium, exposing the surface of the cerebrum, and affording a view of the pulsation of the vessels of the brain. With the aid of the cardiograph, the sphygmograph, the hydrosphygmograph, and the plethosphygmograph, it became possible to register the circulation of the blood in the brain, and to compare that portion of its course with the coincident circulation in other parts of the body. It was thus shown that every increase of emotional or intellectual activity was attended by an increase in the activity of the cerebral circulation. This increase was procured at the expense of other portions of the body, which exhibited a coincident reduction in the amount of blood received from the heart. The occurrence of sleep caused a diminution in the number of respirations, and a fall of six or eight beats in the pulse. The volume of the brain and its temperature were at the same time slightly reduced, through the diversion of blood from the head to other regions of the body. The consequent dilatation of the vessels in the extremities was readily demonstrated by the use of the plethosphygmograph. The extreme sensitiveness of the nervous centers was further illustrated by the fact that if, during sleep, a ray of light were directed upon the eyelids, or if any organ of sense were moderately excited without waking the patient, his respiration was at once accelerated; the heart began to beat more rapidly, the vessels of the extremities contracted, and the blood flowed more freely into the brain. Similar results accompanied the act of dreaming. The return of full consciousness on waking was followed by an immediate increase in the activity of the intra-cranial circulation.

The extreme susceptibility of the brain to influences proceeding from artificial disturbances in the circulation, was exhibited in the case of one of Professor Mosso’s patients. By compression of the carotid arteries, unconsciousness was induced, and an attack of convulsions was aroused. In no other part of the body can a corresponding disturbance of function be so quickly produced by similar means. A limb may be rendered bloodless for nearly half an hour, by the application of an elastic bandage, and yet its sensory nerves will remain capable of transmitting impressions from the periphery. But in this case, compression of the carotids for only eight seconds was sufficient to abolish consciousness and to excite convulsive movements.

In all such observations it is worthy of note that the nervous impression is the primary event so long as artificial disturbances are not intruded. The changes of blood-pressure and circulation were invariably secondary to the excitement of nerve tissue. Sleep, therefore, must be regarded as the cause, rather than the consequence, of the so-called cerebral anÆmia which obtains in the substance of the brain during repose. This condition of “anÆmia” is nothing more than the relatively lower state of circulation which may be remarked in every organ of the body during periods of inactivity. Every impression upon the sensory structures of the brain occasions a corresponding liberation of motion in those structures. The movement thus initiated arouses the vaso-dilator nerves of the cerebral vessels and excites the vaso-constrictor nerves of all other portions of the vascular apparatus. Hence the superior vascularity of the brain so long as the organs of sense are fresh and receptive. Hence the diminishing and varying vascularity of the different departments of the brain as sleep becomes more or less profound. These modifications of the brain and of its circulation are well illustrated by the effects of a moderate degree of cold applied to the cutaneous nerves of the body, as not unfrequently happens when the night grows cool towards morning. The disturbance of the sensitive nerves of the skin is transmitted to the brain. The excitement of this organ causes dilatation of its vessels, and increased irritability of the cortical instrument of perception. This becomes the starting point for the projection of impulses upon the field of consciousness, producing dreams, or even a complete awakening from sleep.

The cause of sleep must, therefore, be sought in the molecular structure of the brain, rather than in fluctuations of the blood-current. In the present state of our knowledge it must be negatively represented as the consequence of a deficiency in the amount of movable oxygen in the nervous tissue. This deficiency may be the result of immaturity, as in the foetus, or in the new-born infant; or it may result from the accumulation of an excess of the waste-products of intra-molecular oxidation during functional activity—products which hinder the further passage of oxygen into stable combination with the oxidizable elements of protoplasm. Sleep thus produced differs from the artificial sleep induced by narcotic drugs, in the fact that its cause is self-generated by the instrument of thought, while narcotic stupor is caused by the intrusion of substances derived from without—substances which, like the natural refuse of the living cells, more or less completely hinder the processes of oxygenation and oxidation within the tissues of the body. Hence the states of healthy sleeping and waking must necessarily be self-limited and regularly successive; while the state of narcotism is purely accidental, and its duration exactly corresponds with the variable length of the period during which the body may remain impregnated with the hypnotic agent.

Certain morbid forms of sleep further illustrate its dependence upon the persistence of depression in the functional activity of the brain. For some persons this seems to constitute their normal condition. They are either excessively fat, red-faced, and soaked with beer, or they are pale, anÆmic, and pulpy, with flabby muscles and a feeble circulation. They fall asleep whenever left to themselves, and never seem to be fully aroused to active life. The fat boy who figures so amusingly in The Pickwick Papers, furnishes a life-like picture of this variety of somnolence.

The introductory stage of the eruptive fevers is often characterized by somnolence. It also frequently appears as the forerunner of coma in the various diseases which terminate in unconsciousness and death. A singular example of this has been observed among the negro inhabitants of the Atlantic coast of tropical Africa. The disorder is known to English writers as sleeping dropsy; by the French it is called maladie du sommeil. It is characterized by daily paroxysms of somnolence which tend to become more and more continuous and profound until they are finally merged in fatal coma. For our knowledge of the disease we are chiefly indebted to the description by Clark,[23] an English surgeon who practised in Sierra Leone, and to the monograph by Guerin,[24] a French naval surgeon, who had enjoyed exceptional opportunities for observation among the laborers who had been carried from Africa to the island of Martinique. Similar cases have been occasionally reported in other regions of the world, but it is among the Africans that it has been principally remarked. The onset of the malady is gradual, commencing with a slight frontal headache. After a few days a disposition to sleep after meals is noted. This becomes increasingly urgent, and the intervals of sleep are prolonged until at length the patient becomes continually soperose. The waking periods are marked by a sluggish state of the intellectual faculties. The pulse is not accelerated, and it remains full and soft. The veins of the sclerotic are turgid, and the eyeball seems unusually prominent. The temperature does not increase, but rather tends to diminish its figure. The skin is dry and moderately cool. The tongue continues moist, and is covered with a white fur. The bowels and the bladder are regularly emptied, and the appetite persists with considerable vigor. Finally, the patient becomes completely comatose, and dies quietly. Sometimes, however, the evolution of the disease is less tranquil. Epileptiform convulsions, followed by progressively deepening periods of coma, interrupt its course, and a continuous muscular agitation marks the closing scene. At the same time the pulse grows weaker and more frequent until its movements cease in death. Recovery is almost unknown, though the duration of the disease often varies from three months to a year or longer. Examination of the body after death yields very negative results; the sinuses and larger vessels of the brain are engorged with blood, but no evidence of inflammation is anywhere apparent. The other organs present no pathological alterations whatever. These observations seem to indicate that the disease originates in some form of general blood-poisoning, rather than in any local inflammation or degeneration. Dr. Clark has called attention to an enlargement of the cervical glands as a feature of the malady. According to Dr. G. H. Bachelder,[25] the native physicians cure the disease by extirpation of the affected glands. He has also observed an initial lesion in the nasal mucous membrane. If this be confirmed, the malady will take its place among the forms of somnolence produced by infection of the blood.

Between the profound unconsciousness of natural sleep and coma may be placed the distinction that the one is always the consequence of healthy physiological processes, while the other is always the result either of injury, of disease, or of some form of intoxication. Comatose unconsciousness may be the result of cerebral compression caused by injury of the head, or by the presence of an inflammatory exudation. Intra-cranial tumors, embolisms, thrombi, diseases of the cerebral arteries, and degenerations of the brain,—in short, every morbid change of which the liquids and the solids within the cranium are capable—may become the cause of coma. ToxÆmic conditions of the circulating fluids of the body may benumb the brain with comatose sleep. Few diseases, therefore, exist without the possibility of coma as one of their consequences—a coma which, however, must not be confounded with the genuine sleep which sometimes occupies the larger part of convalescence from acute illness. During such convalescence there is a reversion to the infantile type of nutrition with all its need of prolonged and frequent periods of repose. Like normal sleep, the comatose condition admits considerable variation of intensity. The patient may sometimes be partially roused, as from the coma of alcoholic intoxication, and he may finally recover complete consciousness; but very often the reverse is the fact. The coma deepens into paralysis of the respiratory centres, and death concludes existence without the slightest manifestation of sensibility or intelligence.

Lethargy is a pathological variety of sleep, in which the repose of the body is even more complete than in coma. The victim of coma often presents a countenance suffused with blood; the pulse beats vigorously, and respiration may become stertorous. But in lethargy the abolition of bodily movement is almost absolute. In the milder forms of this disorder the patient may be partially roused, so as to attempt an answer when addressed, appearing like a person in very deep sleep; but in the majority of cases he remains insensible, unconscious, and utterly irresponsive to ordinary forms of irritation. Respiration and circulation are reduced to a minimum, and may, even for a time, become imperceptible. Uncomplicated with hysteria, the disorder is rapidly fatal, but, according to Rosenthal,[26] hysterical lethargy is never mortal.

Many examples of this disease have been afforded by the records of apparent death. I am well acquainted with a lady who, in early childhood, had been laid out for burial at the supposed termination of some infantile illness. Her mother alone insisted that the child was still alive. After some time spent in weeping and expostulation, she applied a blister to the thorax of the babe. This soon excited evidences of painful irritation, followed by a complete recovery. Still more instructive is the case, narrated by Rosenthal,[27] of a young woman, twenty-four years of age, who, in consequence of violent emotional excitement, became unconscious, and presented no signs of life, though tested by placing a mirror before the mouth, and by dropping melted sealing-wax upon the skin. On raising her eyelids, the pupils gave no reaction to light; the limbs remained perfectly flaccid, and the radial arteries were pulseless. Careful auscultation, however, detected a very feeble and intermittent sound in the cardiac region. The walls of the chest exhibited no movement, but the lateral surfaces of the abdomen presented a slow and almost imperceptible oscillation. Gentle faradization of the muscles and nerves of the face, arm, and hand, excited definite muscular contractions. By this method Rosenthal became satisfied that, although the patient had remained for thirty-two hours in this condition, she was only apparently dead. In fact, after continuing forty-four hours in a state of suspended animation, she awoke spontaneously, made a rapid recovery, and seemed to enjoy as comfortable health as an excitable, nervous temperament would permit.

Certain authors make a distinction between lethargy and apparent death; but the difference is one of degree rather than of kind. The movements of respiration and of circulation, though greatly enfeebled, are readily observed in ordinary forms of lethargy; but in apparent death the pulse can no longer be discovered, and nothing more than the faintest sound can be distinguished in the region of the heart. It, therefore, becomes important to have within reach a crucial test of the persistence of general vitality. Such evidence, according to Rosenthal, is furnished by the faradaic current. Within two or three hours after actual death, the muscles cease to respond to the induced current; but in apparent death this form of electro-muscular contractility never disappears. Every other test that has been proposed has failed under certain circumstances. Observation of the changes in muscular temperature during electrical excitation is a method better adapted to the laboratory than for clinical practice.

Lucid lethargy.—In certain cases of apparent death the patient exhibits all the external appearance of suspended animation, but the power of conscious perception does not cease. The senses of sight and hearing remain, and are, perhaps, intensified by inhibition of the power of voluntary movement. The sufferer sees and hears; perception, emotion, memory, the power of reasoning, judgment, volition, all persist. Only the power of executing voluntary movements is lacking.

The victims of this variety of apparent death are usually women, or men who are characterized by a feminine nervous organization. Great mental excitement, fatigue, semi-starvation, and exhausting diseases, are the principal exciting causes of the event. The following case, related by my friend, Dr. P. S. Hayes, of Chicago, illustrates the phenomena of lucid lethargy. The patient was a female physician, about thirty years of age, unmarried, and consumptively inclined. During the course of a long and wearisome hospital service, she was prostrated with typhoid fever. Placing herself under the immediate care of my informant, she was also attended by several of the most eminent physicians in the city. After a long and exhausting illness she appeared to be dying. In the presence of her physician, and surrounded by her relatives, she ceased to breathe. The pulse stopped, life seemed to have gone out. Bottles of hot water were applied to the limbs, and various restoratives were employed. After a considerable time she began again to breathe, and a gradual recovery followed. During the whole time of apparent death, consciousness had been preserved. She seemed to be looking down from a point above her bed; she could see the doctor feeling for her pulse, and was grieved by the sorrow of her friends. Ordinary sensation was temporarily suspended, and she could not distinguish the contact of the hot-water bottles that were applied to her limbs, though actually scalded by their excessive heat. Borne upon the wings of an excited imagination, she thought herself permitted to look into heaven, but was not suffered to enter its gates. In this exaltation of the imagination the reasoning faculties also shared, so that certain philosophical problems which had previously baffled her intellect were now perfectly comprehensible, and the memory of their solution persisted after recovery.

Many similar narratives have been duly authenticated, but the limits of the present chapter will not permit a discussion which properly belongs to an investigation of the phenomena of trance. The important fact for present consideration is the persistence of conscious life, despite the appearance of death. In this preservation of consciousness, notwithstanding the temporary suspension of certain kinds of sensibility and the power of voluntary motion, may be discovered a relationship between the phenomena of lucid lethargy and various disturbances of sleep, which will be considered in a succeeding chapter.