Popular Science Monthly/Volume 1/August 1872/The Physiology of Sleep
|THE PHYSIOLOGY OF SLEEP.|
"THE twinkling of oblivion," as Wordsworth exquisitely defines the phenomenon of sleep, has, from the time of Hippocrates to the present hour, engaged the attention of thoughtful minds. Poets have found in the phenomenon subject-matter for some of the most perfect of their works. Menander exalts sleep as the remedy for every disease that admits of cure; Shakespeare defines it, "The birth of each day's life, sore labor's bath;" Sir Philip Sidney designates it, "The poor man's wealth, the prisoner's release;" and wearied Dryden sings of it—
Oh! peace of mind, repairer of decay,
Whose balms renew the limbs to labors of the day."
As to the philosophers and the physicians who have said and written on sleep, I dare hardly think of them, lest I should commit myself to an historical volume instead of a short physiological essay; so I leave them, except such as are simply physiological, and proceed on my way.
Perfect sleep is the possession, as a rule, of childhood only. The healthy child, worn out with its day of active life, suddenly sinks to rest, sleeps its ten or twelve hours, and wakes, believing, feeling, that it has merely closed its eyes and opened them again; so deep is its twinkle of oblivion. The sleep in this case is the nearest of approaches to actual death, and at the same time presents a natural paradox, for it is the evidence of strongest life.
During this condition of perfect sleep, what are the physiological conditions of the sleeper? Firstly, all the senses are shut up, yet are they so lightly sealed that the communication of motion by sound, by mechanical vibration, by communication of painful impression, is sufficient to unseal the senses, to arouse the body, to renew all the proofs of existing active life. Secondly, during this period of natural sleep the most important changes of nutrition are in progress; the body is renovating, and, if young, is actually growing; if the body be properly covered, the animal heat is being conserved and laid up for expenditure during the waking hours that are to follow; the respiration is reduced, the inspirations being lessened in the proportion of six to seven as compared with the number made when the body is awake; the action of the heart is reduced; the voluntary muscles, relieved of all fatigue and with the extensors more relaxed than the flexors, are undergoing repair of structure and recruiting their excitability; and the voluntary nervous system, dead for the time to the external vibration, or as the older men called it "stimulus" from without, is also undergoing rest and repair, so that when it comes again into work it may receive better the impressions it may have to gather up, and influence more effectively the muscles it may be called upon to animate, direct, control.
Thirdly, although in the organism during sleep there is suspension of muscular and nervous power, there is not universal suspension; a narrow, but at the same time safe, line of distinction separates the sleep of life from the sleep of death. The heart is a muscle, but it does not sleep, and the lungs are worked by muscles, and these do not sleep; and the viscera which triturate and digest food are moved by muscles, and these do not sleep; and the glands have an arrangement for the constant separation of fluids, and the glands do not sleep; and all these parts have certain nerves which do not sleep. These all rest, but they do not cease their functions. Why is it so?
The reason is, that the body is divided into two systems as regards motion. For every act of the body we have a system of organs under the influence of the will, the voluntary, and another system independent of the will, the involuntary. The muscles which propel the body, and are concerned in all acts we essay to perform, are voluntary; the muscles, such as the heart and the stomach, which we cannot control, are involuntary. Added to these are muscles which, though commonly acting involuntarily, are capable of being moved by the will: the muscles which move the lungs are of this order, for we can if we wish suspend their action for a short time or quicken it; these muscles we call semi-voluntary. In sleep, then, the voluntary muscles sleep, and the nervous organs which stimulate the voluntary muscles sleep; but the involuntary and the semi-voluntary muscles and their nerves merely rest: they do not veritably sleep.
This arrangement will be seen, at once, to be a necessity, for upon the involuntary acts the body relies for the continuance of life. In disease the voluntary muscles may be paralyzed, the brain may be paralyzed, but, if the involuntary organs retain their power, the animal is not dead. Sir Astley Cooper had under his care a man who had received an injury of the skull causing compression of the brain, and the man lay for weeks in a state of persistent unconsciousness and repose; practically he slept. He did not die, because the involuntary system remained true to its duty; and, when the great surgeon removed the compression from the brain of the man, the sleeper woke from his long trance and recovered. Dr. Wilson Philip had a young dog that had no brain, and the animal lay in profound insensibility for months, practically asleep; but the involuntary parts continued uninfluenced, and the animal lived and, under mechanical feeding, grew fat. Flourens had a brainless fowl that lived in the same condition. It neither saw nor heard, he says, nor smelled nor tasted nor felt; it lost even its instincts; for however long it was left to fast, it never voluntarily ate; it never shrunk when it was touched, and, when attacked by its fellows, it made no attempt at self-defence, neither resisting nor escaping. In fine, it lost every trace of intelligence, for it neither willed, remembered, felt, nor judged: yet it swallowed food when the food was put into its mouth, and fattened. In these cases, as in that of the injured man, the involuntary systems sustained the animal life. It is the same in sleep.
When we look at these phenomena, as anatomists, we find a reason for them in structure and character of parts. The involuntary muscles have a special anatomical structure; and the nervous organism that keeps the involuntary muscles in action is a distinct organism. There are, briefly, two nervous systems: one locked up in the bony cavity of the skull and in the bony canal of the spine, with nerves issuing therefrom to the muscles; and another lying within the cavities of the body, with nerves issuing from it to supply all the involuntary muscles. The first of these systems, consisting of the brain, the spinal cord, and the nerves of sense, sensation, and motion, is called the cerebro-spinal or voluntary system of nerves; the second, consisting of a series of nervous ganglia with nerves which communicate with the involuntary muscles and with nerves of the voluntary kind, is called, after Harvey the vegetative, after Bichat the organic system: a sketch of this organic system is depicted in the accompanying diagram.
In sleep, the cerebro-spinal system sleeps; the organic system retains its activity. Thus in sleep the voluntary muscles and parts fail to receive their nervous stimulation; but the involuntary receive theirs still, and under it move in steady motion; while the semi-voluntary organs also receive sufficient stimulation to keep them in motion.
Of all the involuntary organs, the heart, which is the citadel of motion, is most protected. To itself belongs a special nervous centre, that which feeds it steadily with stimulus for motion; from the cervical ganglia of the organic nervous system it receives a second or supplementary supply; and from the brain it receives a third supply, which, passive under ordinary circumstances, can under extraordinary circumstances become active and exert a certain controlling power. Then the arteries which supply the heart with blood are the first vessels given off from the great feeding arterial trunk, and the veins of the heart winding independently round it empty their contents direct again into it. Thus is the heart the most perfect of independencies: thus during sleep and during wakefulness it works its own course, and, taking first care of itself in every particular, feeds the rest of the body afterward; thus, even when sleep passes into death, the heart in almost every case continues its action for some time after all the other parts of the organism are in absolute quiescence; thus, in hibernating animals, the heart continues in play during their long somnolence; and, thus, under the insensibility produced by the inhalation of narcotic gases and vapors, the heart sustains its function when every other part is temporarily dead. Next the heart in independent action is the muscle called the midriff or diaphragm; and, as the diaphragm is a muscle of inspiration, the respiratory function plays second to the circulatory, and the two great functions of life are, in sleep, faithfully performed. In sleep of illness bordering on sleep of death, how intently we watch for the merest trace of breath, and augur that, if but a feather be moved by it or a mirror dimmed by it, there is yet life!
In natural sleep, then, sleep perfect and deep, that half of our nature which is volitional is in the condition of inertia. To say, as Blumenbach has said, that in this state all intercourse between mind and body is suspended, is more perhaps than should be said, the precise limits and connections of mind and body being unknown. But certainly the brain and spinal cord, ceasing themselves to receive impressions, cease to communicate to the muscles they supply stimulus for motion, and the muscles under their control, with their nerves, therefore sleep. And so, to the extent that the acts of the brain and cord and their nerves are mental, and the acts or motions of the voluntary muscles are bodily acts, to that extent, in sleep, the intercourse between the mind and the body is suspended.
In sleep the condition of the involuntary muscles and of the voluntary nervous system is, we must assume, in some manner modified, since these organs are transformed from the active into the passive state. Respecting the condition of the muscles in sleep, no study of a systematic sort has been carried out, but in relation to the brain there has been much thoughtful study, upon which many theories have been founded.
The older physiologists regarded sleep as due to the exhaustion of the nervous fluid; during sleep, they held, this fluid accumulates in the brain; and, when the brain and the other centres and nerves of the cerebro-spinal system are, to employ a common expression, recharged, the muscles are stimulated and the body awakes; the brain prepared to receive external impressions and to animate the muscles, and the muscles renovated and ready to be recalled into activity. This theory held its ground for many years, and, perhaps, still there are more believers in it than in any other. It fails to convince the skeptical because of its incompleteness, for it tells nothing about the nature of the presumed nervous fluid, and we know nothing as yet about this fluid. The primary step of the speculation is consequently itself purely hypothetical.
Another theory, that has been promulgated, is that sleep depends on the sinking or collapse of the laminæ of the cerebellum or little brain. This theory is based on the experiment that compression of the cerebellum induces sleep; but the argument is fallacious, because pressure on the larger brain, or cerebrum, is followed by the same result. The theory of pressure has been proposed again in a different way; it has been affirmed that the phenomenon of sleep is caused by the accumulation of fluids in the cavity of the cranium, and by pressure, resulting from this accumulation, on the brain as a whole. We know well that pressure upon the brain does lead to an insensible condition resembling sleep, and in some instances, in which the skull has been injured and an artificial opening through it to the brain has been formed, pressure upon the exposed surface has led to a comatose condition. I once myself saw a case of this nature. But the evidence against this explanation is strong, because the sleeping brain has been observed to be pale and too free of blood to convey any idea of pressure.
In opposition to the pressure theory, Blumenbach contended that sleep is due to a diminished flow and impulse of blood upon the brain, for he argued the phenomenon of sleep is induced by exhaustion, and particularly by exhaustion following upon direct loss of blood. Recently Mr. Arthur Durham, in a very able communication, has adduced a similar view, and the general conclusion now is, that during sleep the brain is really supplied with less blood than in waking hours.
To account for the reason why the brain is less freely fed with blood in sleep, it has been surmised that the vessels, the arteries, which feed the brain, and which for contractile purposes are supplied with nerves from the organic nervous system, are, under their nervous influence, made to close so that a portion at least of the blood which enters through them is cut off on going to sleep. This view, however, presupposes that the organic nervous centres, instead of sharing in the exhaustion incident to labor, put forth increased power after fatigue, an idea incompatible with all we know of the natural functions.
Carmichael, an excellent physiologist, thought that sleep was brought on by a change in the assimilation of the brain, and by what he called the deposition of new matter in the organ, but he offered no evidence in proof: while Metcalfe, one of the most learned physicists and physicians of our time, maintained that the proximate cause of sleep is an expenditure of the substance and vital energy of the brain, nerves, and voluntary muscles, beyond what they receive when awake, and that the specific office of sleep is the restoration of what has been wasted by exercise: the most remarkable difference between exercise and sleep being, that during exercise the expenditure exceeds the income; whereas during sleep the income exceeds the expenditure. This idea of Metcalfe's expresses, probably, a broad truth, but it is too general to indicate the proximate cause of sleep, to explain which is the object of his proposition.
My own researches on the proximate cause of sleep—researches which of late years have been steadily pursued—lead me to the conclusion that none of the theories as yet offered account correctly for the natural phenomenon of sleep; although I must express that some of them are based on well-defined facts. It is perfectly true that exhaustion of the brain will induce phenomena so closely allied to the phenomena of natural sleep, that no one could tell the artificially-induced from the natural sleep; and it is equally true that pressure upon the brain will also lead to a state of sleep simulating the natural. For example, in a young animal, a pigeon, I can induce the deepest sleep by exposing the brain to the influence of extreme cold. I have had a bird sleeping calmly for ten hours under the local influence of cold. During this time the state of the brain is one of extreme blood lessness, and, when the cold is cautiously withdrawn and the brain is allowed to refill gently with blood, the sleep passes away. This is clear enough, and the cold, it may be urged, produces contraction of the brain-substance and of the vessels, with diminution of blood, and with sleep as the result. But if, when the animal is awaking from this sleep induced by cold, I apply warmth, for the unsealing of the parts, a little too freely, if, that is to say, I restore the natural warmth too quickly, then the animal falls asleep again under an opposite condition; for now into the relaxed vessels of the brain the heart injects blood so freely, that the vessels, in like manner as when the frozen hand is held near the fire, become engorged with blood, there is congestion, there is pressure, and there is sleep.
The same series of phenomena from opposite conditions can be induced by narcotic vapors. There is a fluid called chloride of aonyl, which, by inhalation, causes the deepest sleep; during the sleep so induced, the brain is as bloodless as if it were frozen. There is an ether called methylic, which, by inhalation, can be made to produce the deepest sleep; during this sleep the vessels of the brain are engorged with blood.
We are therefore correct in supposing that artificial sleep may be induced both by removal of blood from the brain, and by pressure of blood upon the brain; and in the facts there is, when we consider them, nothing extraordinary. In both conditions, the natural state of the brain is altered; it cannot, under either state, properly receive or transmit motion; so it is quiescent, it sleeps. The experimental proof of this can be performed on any part of the body where there are nerve-fibre and blood-vessel; if I freeze a portion of my skin, by ether-spray, I make it insensible to all impression—I make it sleep; if I place over a portion of skin a cupping-tube, and forcibly induce intense congestion of vessels, by exhausting the air of the tube, I make the part also insensible—I make it sleep.
The two most plausible theories of sleep—the plenum and the vacuum theories I had nearly called them—are, then, based on facts; but still I think them fallacious. The theory that natural sleep depends on pressure of the brain from blood is disproved by the observations that have been made of the brain during sleep, while the mechanism of the circulation through the brain furnishes no thought of this theory as being possibly correct. The theory that sleep is caused by withdrawal of blood from the brain, by contraction of its arterial vessels, is disproved by many considerations. It presupposes that at the time when the cerebro-spinal nervous system is most wearied the organic system is most active; and it assumes that the great volume of blood which circulates through the brain can be cut off without evidence of increased volume of blood and tension of vessel in other parts of the body, a supposition directly negatived by the actual experiment of cutting off the blood from the brain.
There is another potent objection applicable to both theories. When sleep is artificially induced, either by subjecting the brain to pressure of blood, or to exhaustion of blood, the sleep is of such a kind that the sleeper cannot be roused until the influence at work to produce the sleep is removed. But, in natural sleep, the sleeper can always be roused by motion or vibration. We call to a person supposed to be sleeping naturally, or we shake him, and if we cannot rouse him we know there is danger; but how could these simple acts remove pressure from the brain, or relax the contracted vessels feeding the brain?
These two theories set aside, the others I have named need not trouble us; they are mere generalizations, interesting to read, worthless to pursue. Know we then nothing leading toward a solution of the question of the proximate cause of sleep? I cannot say that, for I think we see our way to something which will unravel the phenomenon; but we must work slowly and patiently, and as men assured that, in the problem we are endeavoring to solve, we are dealing with a subject of more than ordinary importance. I will try to point out the direction of research.
I find that to induce sleep it is not necessary to produce extreme changes of brain-matter. In applying cold, for example, it is not necessary to make the brain-substance solid in order to induce stupor, but simply to bring down its temperature ten or twelve degrees. I find also that very slight direct vibrations, concussions, will induce stupor; and I find that, in animals of different kinds, the profoundness of sleep is greater in proportion as the size of the brain is larger. From these and other facts, I infer that the phenomenon of natural sleep is due to a molecular change in the nervous structure itself of the cerebro-spinal system, and that in perfect sleep the whole of the nervous structure is involved in the change—the brain, the cord, the nerves; while in imperfect sleep only parts of this nervous matter are influenced. This is in accord with facts, for I can by cold put to sleep special parts of the nervous mass without putting other parts to sleep. In bad sleep we have the representation of the same thing in the restlessness of the muscles, the half-conscious wakings, the dreams.
Suppose this idea of the change of nervous matter to be true, is there any clew to the nature of the change itself? I think there is. The change is one very closely resembling that which occurs in the solidification of water surcharged with a saline substance, or in water holding a hydrated colloid, like dialyzed silica, in trembling suspension. What is, indeed, the brain and nervous matter? It is a mass of water made sufficiently solid to be reduced into shape and form, by rather less than twenty per cent, of solid matter, consisting of albuminous substance, saline substance, fatty substance. The mechanism for the supply of blood is most delicate, membranous; the mechanism for dialysis or separation of crystalloidal from colloidal substance is perfect, and the conversion of the compound substance of brain from one condition of matter to another is, if we may judge from some changes of water charged with colloidal or fatty substances, extremely simple. I do not now venture on details respecting this peculiarly interesting question, but I venture so far as to express what I feel will one day be the accepted fact, that the matter of the wakeful brain is, on going to sleep, changed, temporarily, into a state of greater solidity; that its molecular parts cease to be moved by external ordinary influences, by chemical influences; that they, in turn, cease to communicate impressions, or, in other words, to stimulate the voluntary muscles; and that then there is sleep which lasts until there is resolution of structure, whereupon there is wakefulness, from renewed motion in brain-matter, and renewed stimulation of voluntary muscle, through nerve.
The change of structure of the brain which I assume to be the proximate cause of sleep is possibly the same change as occurs in a more extreme degree when the brain and its subordinate parts actually die. The effects of a concussion of the brain from a blow, the effects of a simple puncture of nervous matter in centres essential to life—as the point in the medulla oblongata which Flourens has designated the vital point—have never been explained, and admit, I imagine, of no explanation except the change of structure I have now ventured to suggest.
Here, for the moment, my task must end. My object has been to make the reader conversant with what has been said by philosophers upon the subject of sleep and its proximate cause, and to indicate briefly a new Line of scientific inquiry. I shall hope on some future occasion to be able to announce further and more fruitful labor.