Popular Science Monthly/Volume 2/November 1872/On the Functions of the Brain
|ON THE FUNCTIONS OF THE BRAIN.|
THE first task of physiology was to localize the functions of life in the various organs of the body which serve as their instruments. Thus digestion was assigned to the stomach, circulation to the heart, respiration to the lungs; thus, too, the seat of intelligence and thought was placed in the brain. Still, with regard to the latter organ, a reservation was thought proper, excluding the idea that the metaphysical expression of the intellectual and moral powers was the manifestation, simply and merely, of the cerebral function. Descartes, who is to be classed among the promoters of modern physiology, because he thoroughly understood that the explanation of vital phenomena must depend on the general laws of physics and of mechanism, expressed himself very plainly on this matter. Adopting Galen's ideas on the formation of "animal spirits" in the brain, he assigns them the task of distribution by means of the nerves throughout the animated machine, so as to carry to each of the parts the impulse needed for its special activity. Yet, above and apart from this physiological function of the brain, Descartes admits the soul, which gives man the faculty of thinking: it was supposed to have its seat in the pineal gland, and to direct those "animal spirits" which issue from and are subject to it.
Descartes' opinions as to the function of the brain would not bear the slightest examination by modern physiology; his explanations, founded on imperfect anatomical knowledge, produced nothing but hypotheses marked by the coarsest mechanical conceptions. Yet they have an historic value for us, in the proof that this great philosopher recognized two things in the brain: first, a physiological mechanism; and then, above and beyond that, the thinking faculty of the soul. These ideas are nearly the same with those that afterward prevailed among many philosophers and some naturalists; the brain, in which the most important functions of the nervous system are performed, was for them not the real organ of thought, but simply the substratum of intelligence. Indeed, the objection was often enough expressed, that the brain forms a physiological exception to all the other organs of the body, in that it is the seat of metaphysical manifestations, which the physiologist has no concern with. It was perceived how digestion, respiration, movement, etc., could be referred to the phenomena of mechanism, of physics, and chemistry; but it was not allowed that thought, intelligence, and will, could be subjected to like explanation. There is, it was said, a chasm between the organ and the function, because the question is about metaphysical phenomena, and not at all about physico-chemical mechanism. De Blainville, in his lectures on zoology, laid great stress on the distinction between the organ and the substratum. "In the organ," he said, "there is a visible and necessary connection between anatomical structure and function; in the heart, the organ of circulation, the form and arrangement of valves and orifices account perfectly for the circulation of the blood. In the substratum, nothing like this is observable; the brain is the substratum of thought; thought has its seat in the brain, but it cannot be inferred from the brain's anatomy." Such considerations served as a foundation for the belief that, in cases of insanity, the reason might be affected essentially, as it was termed; that is, without the existence of any lesion in the substance of the brain. Even the converse was asserted, and cases are cited in physiological treatises of the unimpaired manifestation of intelligence in persons with softened or indurated brains. The progress of modern science has destroyed all such doctrines; yet it must be admitted that those physiologists who have drawn from the most delicate recent researches into the structure of the brain the conclusion that thought must be localized in a particular substance, or in nerve-cells of a determinate form and order, have equally failed to solve the problem, since they have done nothing more, in reality, than to oppose materialistic theories to other spiritualistic theories.
From what has been said, I shall draw the only conclusion which legitimately results; namely, that the mechanism of thought is unknown to us—a conclusion with which every one will probably agree. None the less the fundamental question I have suggested exists; for what concerns us is to know whether our present ignorance on this subject is a relative ignorance which will vanish with the progress of science, or an absolute ignorance in the sense of its relating to a vital problem which must forever remain beyond the ken of physiology. For myself, I reject the latter opinion, because I deny that scientific truth can thus be divided into fractions. How, indeed, can one understand that it is permitted to the physiologist to succeed in explaining the phenomena that occur in all the organs of the body, except a part of those that occur in the brain? Such distinctions cannot exist among vital phenomena. Unquestionably they present very different degrees of complexity, but they are all alike in being either soluble or insoluble by our examination; and the brain, marvellous as those metaphysical manifestations that take place in it appear to us, cannot form an exception among the other bodily organs.
From a physiological point of view, those metaphysical phenomena of thought, consciousness, and intelligence, which serve for the various manifestations of the human soul, are nothing but ordinary vital phenomena, and can result from nothing but the action of the organ that expresses them. We shall show that, in fact, the physiology of the brain, like that of all the other bodily organs, is deduced from anatomical observations, from experiments conducted physiologically, and from the teachings of pathological anatomy.
In its anatomical development the brain follows the general law; that is, it increases in volume whenever the functions which it controls increase in energy. In the graduated orders of animals we find the brain gain in development in proportion to the greater manifestation of intelligence; and in man, with whom the phenomena of mind have reached their highest expression, the cerebral organ presents the largest volume. The intelligence of the various animals can be readily inferred from the shape of the brain, and the number of creases or folds that extend its surface. But not only does the outward appearance of the brain change with the modification of its functions; it presents in its inner structure also a complexity that increases with the variety and intensity of the mental manifestations. As regards the texture of the brain, we are long past the days of Buffon, who considered the brains, as he contemptuously called them, a mucous substance of no importance. The advance of general anatomy and of histology has taught us that the cerebral organ possesses a texture more delicate as well as more complex than that of any other nerve-arrangement. The anatomical elements that make it up are nerve-elements in the shape of tubes and of cells variously joined and interlaced. These elements are alike in all animals as to their physiological properties and histologic character; they differ as to their number, net-work, and connection, in a word their arrangement, which in the brain of various species presents a disposition peculiar to each. In this the brain again follows a general law, for in all organs the anatomical element has fixed characteristics by which it may be known; the completeness of the organ consists chiefly in the arrangement of these elements, which presents in every animal species its own peculiar form. Every organ is in fact, then, an instrument whose constituent elements remain identical, while their grouping grows more and more involved in the same degree as the function itself displays more variety and complexity.
Reflecting, now, on the organic and physico-chemical conditions required for the support of life and the discharge of its functions, we find that they are the same in the brain as in all the other organs. The blood acts on the anatomical elements of all the tissues by supplying their indispensable conditions of nutrition, temperature, and humidity. When a diminished supply of blood flows to any organ, its activity of function declines, and the organ rests; but if the blood is quite cut off, the elementary properties of the tissue slowly change, while at the same time its function perishes. It is precisely the same as to the brain's anatomical elements: as soon as the blood ceases to flow to it, its nerve-properties are affected, as well as its function, which gradually disappears, if the blood remains wholly withheld. A simple modification of the temperature of the blood, in its pressure, is enough to produce grave disturbances in the sensibility, the power of motion, or the will.
All the bodily organs present alternate states of rest and of activity in which the phenomena of circulation differ essentially. Numerous observations, made upon the most different structures, place these facts beyond doubt. When, for instance, we examine the alimentary canal of a fasting animal, we find the mucous membrane that lines the inner face of the stomach and intestines, pale and but little supplied with blood; during digestion, on the contrary, we learn that the same membrane is highly colored, and swollen with the blood which flows energetically into it. These two phases of circulation, in a state of rest and a state of activity, have been brought under direct investigation in the stomach of a living man. All physiologists recollect the story of a young Canadian accidentally wounded by a leaden musket-ball which struck him almost point-blank on the left side. The abdominal cavity was laid open by an immense contused wound, and the stomach, extensively perforated, allowed the food which he had last taken, to escape. The patient was attended by Dr. Beaumont, a surgeon of the United States Army; he recovered, but retained a fistulous wound, opening with a circumference of about an inch and a half through which different substances could be introduced, and the action of the stomach easily examined. Dr. Beaumont, anxious to study this remarkable case, employed the young man as a servant, after the complete restoration of his health and particularly of his digestive powers. He was able to keep him in his service for seven years, during which he made a great number of observations of the highest interest to physiology. On looking into the interior of the stomach while empty of food, the lining membrane could be plainly seen, lying in uneven folds, with its surface of a pale rose-color, motionless, and lubricated by forming whatever but mucus. As soon as articles of food made their way into the stomach, and touched the mucous membrane, its circulation grew rapid and its color lively, while peristaltic movements became evident. The mucous papillae then poured out their gastric juice, a clear and transparent fluid, designed to dissolve the food. On wiping away the mucus that covered the villous membrane, with a sponge or fine cloth, the gastric juice was soon seen reappearing and gathering in little drops that ran along the walls of the stomach like perspiration on the face. What we have just seen as to the mucous membrane is known to occur alike throughout the intestine, and in all the glandular organs connected with the digestive apparatus. The salivary glands and the pancreas, in the intervals of the act of digestion, present a pale and bloodless tissue, the secretions of which are wholly suspended. During the period of digestion, on the contrary, these same glands are swollen with blood, as if inflamed and erectile, while their vessels pour out the secreted fluids abundantly.
Two orders of circulation, then, must be recognized in the organs: one, the general circulation, known since Harvey's day; and the other, local circulation, only discovered and studied in recent times. In the phenomena of general circulation the blood may be said to do nothing more than traverse the parts, to pass from the arteries into the veins; in the phenomena of local, which is the true functional circulation, the blood penetrates all the folds of the organ, and gathers closely about its anatomical elements, to arouse and excite their special mode of activity. The nervous system, sensitive in its action through the vessels, governs all those phenomena of local circulation which attend organic activity; thus, the saliva flows copiously when a sapid substance makes an impression on the nerves of the mucous membrane of the mouth, and the gastric juice forms under the influence of contract between food and the sensitive surface of the stomach. But, for this mechanical excitement of the peripheral nerves of sensation, influencing the organ by reflex action, a purely psychic or cerebral excitement can be substituted. A simple experiment proves this: If a horse is taken while fasting, and the excretory duct of the parotid gland upon the side of the jaw is exposed and divided, nothing flows from it; the gland is at rest. If, now, oats are shown to the animal, or, still better, if, without any thing being shown, a movement is made which leads him to think he is about to have food given him, immediately a continuous flow of saliva issues from the duct of the parotid, and at the same time the tissue of the gland is injected, and becomes the seat of a more active circulation. Dr. Beaumont remarked similar phenomena in his Canadian. The idea of savory food not only solicited a secretion from the salivary glands, but provoked, besides, an immediate flow of blood to the mucous membrane of the stomach.
What we have just said as to the local or functional circulations, applies not only to those secreting organs in which there takes place the separation of a liquid, to the formation of which the blood must more or less give its aid; it rather expresses a phenomenon generally remarked in all the organs, whatever the nature of their function may be. The muscular system, which produces nothing but mechanical work, is in this regard like the glands, which act chemically. At the instant of muscular action the blood circulates with greater activity, which relaxes when the organ begins to rest. The peripheral nervous system, the spinal marrow, and the brain, which serve to manifest the phenomena of innervation and intelligence, are equally subject to this law, as we are about to see.
The relations existing between the phenomena of circulation in the brain and the functional activity of that organ have long remained obscure, owing to mistaken ideas of the conditions of sleep, which is rightly considered the state of rest of the cerebral organ. The ancients suppose that sleep resulted from compression exerted on the brain by the blood when its circulation declined. They imagined that this pressure was chiefly exerted at the back part of the head, at the point where the veined folds of the dura mater unite in a common confluent, which is still called the torcular or compress of Herophilus, from the name of the anatomist who first described it. These conjectural explanations have been handed down to us; and it is only of late years that experiment has succeeded in proving their falsity. In fact, it has been shown by direct experiment that, during sleep, the brain, instead of being congested, is on the contrary pale and bloodless; while in a state of wakefulness the circulation, becoming more active, provokes a flow of blood proportioned to the intensity of cerebral activity. In this respect natural sleep and the anaesthetic sleep of chloroform are alike; in both cases, the brain, sunk into rest or inactivity, presents the same paleness and relative bloodlessness.
The experiment is made in this manner: A part of the bony covering of an animal's skull is carefully removed, and the brain laid bare so as to study the circulation at the surface of this organ. Then chloroform is administered to produce insensibility. In the first exciting stage of the action of the chloroform, the brain is observed to grow congested and to lap over at the edges; but as soon as the stage of anaesthetic sleep is reached, the substance of the brain sinks in and grows paler, presenting a languid movement of capillary circulation, which lasts as long as the state of sleep or cerebral rest continues. For the study of the brain in natural sleep a circular trepan is made on a dog's head, and the piece of bone removed is replaced by a watch-glass carefully adjusted to the exact opening, so as to prevent the irritating action of the air. The animals subjected to the operation survive it; and observations on their brain through this sort of window, while awake and when asleep, prove that when the dog is asleep the brain is always paler, and that a fresh afflux of blood is regularly noticed on his awaking, when the functions of the brain resume their activity. Facts analogous to those observed in animals have been studied directly in the human brain. Upon a person injured by a frightful railroad accident the effect of a considerable loss of brain-substance was examined. The brain was visible over a surface of three by six inches. The patient suffered frequent and severe attacks of epilepsy and coma, during which the brain invariably expanded. Sleep succeeded these attacks, and the cerebral hernia gradually subsided. When the patient awoke, the brain again projected and rose to the level of the surface of the external, bony table. In the case of another person injured in consequence of a fracture of the skull, the cerebral circulation was studied during the administration of anaesthetics. With the first inhalations, the surface of the brain became branchy and filled with blood; the flow of blood and throbbing of the brain increased, and then, at the instant of sleep, its surface subsided by degrees below the opening, while at the same time growing relatively pale and bloodless.
Briefly, then, the brain is governed by the common law that controls blood-circulation in all the organs. By virtue of this law, when the organs are at rest and their action suspended, the circulation in them grows languid; and it increases, on the contrary, as soon as activity is resumed. The brain, I repeat, is no exception to this general law, as had been supposed, for it is now demonstrated that the state of sleep coincides not with congestion, but, on the contrary, with bloodlessness of the brain.
If we seek now to understand the relations that may exist between great activity of blood-circulation and the functional condition of the organs, we shall readily see that this increased flow of the sanguineous fluid corresponds with greater intensity in the chemical alterations going on within the tissues, as also with an exaltation in the phenomena connected with heat which are their necessary and immediate consequence. The production of heat in living beings is a fact established from remote antiquity; but the ancients had erroneous ideas as to the origin of heat: they attributed it to an innate organic power that had its seat in the heart, that ardent centre of ebullition for the blood and the passions. At a later date the lungs were regarded as a sort of furnace to which the mass of the blood repaired successively to gain the heat which circulation was bidden to distribute throughout the body. The advance of modern physiology has proved that all these absolute consignments of vital conditions to special points are chimeras. The sources of animal warmth exist everywhere, and in no region exclusively. It is only through the harmonious functional play of the various organs that the temperature is kept nearly constant in man and the warm-blooded animals. There are, in truth, as many heat-producing centres as there are special organs and tissues, and we are obliged always to connect evolving heat with functional labor of the organs. When a muscle contracts, when a mucous surface or a gland secretes, production of heat invariably takes place at the same time with increased activity in the phenomena of local circulation.
Is the case the same with the nervous system and the brain? Modern experiments forbid us to doubt it. Whenever the spinal marrow and the nerves exhibit sensibility or movement, whenever an intellectual effort takes place in the brain, a corresponding quantity of heat is evolved in it. We must, then, regard heat in the animal economy as a resultant of the organic labor of all the parts of the body; but at the same time it becomes also the principle of activity for each of these parts. This correlation is, above all, indispensable for the brain and the nervous system, which hold all the other vital actions under their control. Experiments have demonstrated that the tissue of the brain exhibits a higher temperature than any other organ of the body. In man and the warm-blooded animals the brain itself produces the heat required for the manifestation of the peculiarities of its tissue. If this were not so, it would infallibly grow cooler, and we should at once see all the functions of the brain become torpid, and intelligence and will perish. This does, in fact, occur in cold-blooded animals, in which the function of heat-production is not energetic enough to sustain the organism in resistance to external causes of refrigeration.
With respect to the organic or physico-chemical conditions of its activity, the brain, then, presents nothing exceptional. If we turn to experiments made upon it by physiologists, we shall find that they have succeeded in analyzing cerebral phenomena in the same way as those of all the other organs. The experimental process usually employed to determine the functions of organs consists in removing them or in destroying them either gradually or suddenly, so as to determine the uses of the organ according to the special disturbances thus caused in vital phenomena. This method of the removal or destruction of organs, which forms a sort of brutal vivisection, has been applied on a great scale to the study of the whole nervous system. Thus, after a nerve is cut, when the parts to which it had been distributed lose their sensibility, we conclude from this that it is one of the nerves of sensation; if it is motion that ceases, we infer thence that we are dealing with one of the nerves of motion. The same method has been applied in examining the functions of the different parts of the encephalic organ, and, though the complexity of the parts has occasioned novel difficulties of execution, the method has yielded results that are not to be contested. Every one has long known that, without the brain, intelligence is not possible, but experiment has discovered exactly the part that is played by each portion of the organ. It teaches us that consciousness, or intelligence properly so called, resides in the cerebral lobes, while the lower portions of the brain contain nervous centres destined for organic functions of a lower kind. This is not the place to describe the special functions of these different sorts of nervous centres which are superposed and in a manner ranged along quite into the spinal marrow; it is enough to say that we owe the knowledge of them to that method of vivisection by organic removal which is adopted in a general way in all physiological inquiries. Here the brain behaves in exactly the same way as all the other bodily organs, in this sense, that every lesion of its substance produces characteristic disturbances in its functions, which always correspond with the mutilation effected.
By means of the cerebral lesions he produces, the physiologist does not stop at the creation of local paralysis, which suspends the action of the will on certain organic instruments; he is able also, by merely disturbing the equilibrium of cerebral action, to produce a suspension of freedom in voluntary motion. Thus, by injuring the peduncles of the cerebellum, and different points of the brain, the experimenter can make an animal move as he chooses, to right or left, forward or back-ward, or can make it turn, sometimes by leaps, sometimes by rotary movement on the axis of its body. The will of the animal persists, but power to guide its motions is gone. In spite of its efforts of will, it moves necessarily in the direction determined by the organic lesion. Pathologists have remarked numerous similar instances in man. Lesions of the peduncles of the cerebellum create rotary movements in men as in animals. Some patients could walk only straight onward. In one case, cruel in its irony, a brave veteran general could only move backward. Therefore the will, which proceeds from the brain, does not take effect on our organs of locomotion themselves; it impresses itself on secondary nervous centres, which need to be kept harmoniously balanced by a perfect physiological equilibrium.
There is another and more delicate experimental method, which consists in introducing into the blood various poisonous substances intended to exert their action upon the anatomical elements of the organs, while these are left undisturbed and kept uninjured. Aided by this method, we can extinguish separately the properties of certain nervous and cerebral elements, in the same way that we can also sever the other organic elements, whether muscular or sanguine. Anæsthetics, for instance, destroy consciousness and depress sensibility, while they leave the power of movement untouched. Curare, on the other hand, destroys the power of movement, and leaves sensibility and will unimpaired; poisons affecting the heart, suspend muscular contractility, and the oxide of carbon destroys the oxidizing properties of the blood-globules, without at all affecting the properties of the nerve-elements. As we see, by this method of investigation or elementary analysis of organic properties, the brain and those phenomena that have their seat in it may also be affected in the same manner as all the other functional instruments of the body.
There is yet a third method of experimenting, which may be called that of experiment by reproduction. This method, to some extent, combines physiological analysis and synthesis, and enables us to establish by evidence and counter-evidence those relations which unite the organ with the function in cerebral manifestations. When the brain of the inferior animals is removed, the function of the organ is necessarily suppressed; but the persistence of life in these beings allows the brain to grow again, and, in proportion as the organ reproduces itself, we observe its functions reappear. The like experiment succeeds in the same way with superior animals, such as birds, in whom intelligence is much more developed. For instance, when the cerebral lobes of a pigeon have been removed, the animal at once loses its senses, and the power of seeking its food. Yet if the animal is artificially fed, it can survive, because its functions of nutrition continue unimpaired so long as their special nervous centres are left unharmed. Little by little the brain renews itself with its particular anatomical elements, and in the degree in which this restoration takes place we observe the animal's use of its senses, and its instincts and intelligence return. Here, I repeat with emphasis, the experiment is complete: there has been as it were both analysis and synthesis of the vital function, because the successive destruction of the different parts of the brain has successively extinguished its different functional manifestations, and because the successive reproduction of the same parts has caused the same manifestations to reappear. It is hardly necessary to add that the same thing happens as to all the other parts of the body which are susceptible of reproduction.
Diseases, which are at bottom nothing but vital perturbations caused by Nature instead of being produced by the hand of the physiologist, affect the brain according to the usual laws of pathology; that is to say, by occasioning functional troubles which always correspond to the nature and seat of the injury. In a word, the brain has its pathological anatomy exactly as all the organs of the economy have, and the pathology of the brain has its special series of symptoms, just as the other organs have theirs. In mental alienation we observe the most remarkable disturbances of the reason, furnishing in their study a rich mine for the researches of the physiologist and the philosopher; but the various forms of lunacy or madness are nothing more than disturbances of the normal function of the brain, and these alterations of function in the cerebral organ, as in all the rest, are combined with invariable anatomical alterations. If, under many circumstances, these are not yet understood, the blame must be laid wholly on the imperfection of our means of investigation. Besides, do we not find that certain poisons, such as opium and curare, paralyze the nerves and the brain, without being able to discover any visible alteration in the nerve-substance? Yet we are sure that such alterations exist; for, to admit the contrary, would be to admit an effect without a cause. When the poison has ceased to act, we find the mental disturbances disappear, and the normal condition return. It is the same when pathological injuries are healed; the trouble in the intelligence ceases, and reason comes back. Pathology here, too, furnishes us with a kind of functional analysis and synthesis, just as may be observed in experiments of reproduction. Disease, in a word, suppresses the function more or less entirely, by changing more or less completely the texture of the organ, and the cure restores the function by reestablishing the normal organic condition.
If the manifestations of the brain's functions were the earliest to attract the attention of philosophers, they will assuredly be the last to receive explanation from physiologists. We believe that the progress of modern science allows us now to approach the subject of the physiology of the brain; but, before beginning the study of the cerebral functions, we must clearly understand our point of departure. In this essay, we have attempted to state only one term of the problem, and to show how untenable is the opinion that the brain forms an exception in the organism, and is the substratum of intelligence instead of being its instrument. This idea is not merely an obsolete conception, but an unscientific one, injurious to the progress of physiology and psychology. Indeed, what sense is there in the notion that any apparatus of Nature, whether in its lifeless or its living domain, can be the seat of a phenomenon without being its instrument? Preconceived ideas clearly have a great influence in discussing the functions of the brain, and a solution is combated by arguments used for the sake of their tendency. Some refuse to allow that the brain can be the organ of intelligence, from fear of being involved by that admission in materialistic doctrines; while others eagerly and arbitrarily lodge intelligence in a round or fusiform nerve-cell, for fear of being charged with spiritualism. For ourselves, we are not concerned about such fears. Physiology tells us that, except in the difference and the greater complexity of the phenomena, the brain is the organ of intelligence in exactly the same way that the heart is the organ of circulation, and the larynx that of the voice. We discover everywhere a necessary bond between the organs and their functions; it is a general principle, from which no organ of the body can escape. Physiology should copy the example of more advanced sciences, and free itself from the fetters of philosophy that would impede its progress; its mission is to seek truth calmly and confidently, its object to establish it beyond doubt or change, without any alarm as to the form under which it may make its appearance.