Popular Science Monthly/Volume 26/April 1885/The Nervous System and Consciousness I
|THE NERVOUS SYSTEM AND CONSCIOUSNESS.|
PROFESSOR OF PSYCHOLOGY AND LOGIC IN THE UNIVERSITY OF CINCINNATI.
IT is the design of these papers to consider some of the more recent experiments and opinions as to the relations between nerve-matter and consciousness. The subject has been much be-written; the final word, however, is not in print, or likely to make its appearance there for some time to come. This is plain enough to anatomist and physiologist, and the general reader may assure himself to the same effect by reference to the proceedings of the Society for Psychical Research. These proceedings offer a considerable number of facts not readily classifiable under any theory now at hand.
There is present and urgent need for a clear statement of the case respecting nerve-matter and consciousness. There is equally urgent need that this statement should be reasoned upon according to the fundamental working of reason—viz., the detection of difference and similarity. It is the proper reward of modern science to have taught that reasoning is the procedure from the known to the unknown by the pathway of resemblance. This resemblance must be experimentally determined and experimentally verified. Dr. Maudsley says, most truly, in his last edition of the "Physiology and Pathology of the Mind": "It will not advance knowledge to identify phenomena of a different kind by giving them the same name; on the contrary, the progress of knowledge lies in following the specializations of development, and in defining differences by a precise use of terms." The characteristic phenomena of nerve-matter and the characteristic phenomena of consciousness should be stated and compared: if these phenomena prove different, they should be described in the light of that difference, and all conclusion as to their origin should be drawn according to that difference. Our first inquiry, then, is concerning the nervous system, as to its parts and functions.
The histological elements of the nervous system are two, the fiber and the cell. The fiber appears under two forms, the medullated and non-medullated. The medullated fiber consists of a central thread or axial band, then a soft substance called the medulla, and, inclosing these, a tubular sheath. The axial band is the essential anatomical element of the fiber. It is an albuminoid substance—that is to say, it is highly unstable in character and complex in structure. The medullary substance is transparent, homogeneous, and strongly refracting, like oil; it consists chiefly of lecithin and cerebrin. The sheath is a lime-substance. The differences in chemical composition between this axial band and its marrow-like inclosure were shown by Lister and Turner in 1859. The band becomes red by a solution of carmine, while the marrow substance is unchanged, and in turn this substance becomes opaque and brown under chromic acid, while the band remains unaltered.
The second element of nerve-matter is the cell. This, in its fully developed condition, is of irregular shape, with strongly refracting granular contents and a distinct nucleus and nucleolus. Many of the cells have one or more prolongations, and are accordingly classed as unipolar, bipolar, or multipolar. One at least of the processes of a multipolar cell is continuous with a fiber and is called the axial-cylinder process. In the cells and in the intercellular substance of central nerve-organs, albuminoid stuff is closely mingled with the other component parts of nerve-matter.
The proportion of solids to water is but twelve per cent in the cells, while it is twenty-five per cent in the fibers. This fact, taken in connection with the much greater provision for distribution of blood to the cell-mass, is strongly confirmatory of Mr. Spencer's opinion that the cells liberate motion by destruction of their substance, and the fibers by isomeric transformation.
The nerve-matter thus described is distributed over the body by two divisions—the great sympathetic and the cerebro-spinal systems. The latter alone concerns us in this paper. The nerve-matter of the cerebro-spinal system is found in the spinal cord and the encephalon. The spinal cord is a nearly cylindrical mass, from fifteen to eighteen inches long, and connected at its anterior extremity with the brain.
The cell-matter lies at the center and forms a continuous ganglionic band. The fibers are on the outside, and are divested of their tubular sheath. The central cell-matter of the cord is curiously shaped into two partial crescents, which are connected with one another by cell-substance called the gray commissure. Fig. 1.—Human Nerve-Fibres of different sizes (Kölliker). a, a, a, healthy fibers the largest of which is "dark-bordered"; b, b, fibers altered by exposure. Magnified 350 diameters. Thirty-one pairs of nerves arise from the sides of the spinal cord and supply the entire body, except the head and other parts receiving branches from the cranial nerves. Each nerve leaving the cord contains, at its origin, all the filaments into which it may afterward be divided. Each filament or fiber remains anatomically distinct throughout its course. The fibers in the cord are connected by a commissure called the white commissure, and those on different sides of the cord extend longitudinally and are parallel to each other. Some of these longitudinal fibers, passing from below upward, convey impulses to the cord or brain; others, descending from the brain and higher parts of the cord, transfer motor impulses to muscles. Without presenting more anatomical detail, it is sufficient to say that the halves of the cord are unified by the fibers of the commissures, and that it works as one organ.
The nerve-matter of the encephalon is divided into three principal parts, viz., the medulla oblongata, the cerebellum, and the cerebrum. The medulla is a continuation and enlargement of the spinal cord at its entrance into the cranial cavity. Here the anterior fibers of the cord become the anterior pyramids of the medulla, while the posterior fibers of the cord are called the restiform bodies. Immediately to the side of the pyramids are projections called the olivary bodies. The medulla is about one and one quarter of an inch long, and one inch broad at the widest part. There is cell-matter in the restiform and olivary bodies, a part of this matter being continuous with the cell-matter of the cord, and a part consisting of independent masses. Directly above the medulla is what is known as the pons Varolii. There are masses of cell-matter scattered at irregular intervals through the pons, but it is made up principally of longitudinal and transverse fibers. The longitudinal fibers connect the medulla with the cerebrum, while the transverse fibers unite the halves of the cerebellum. This organ, the cerebellum, is made up of two hemispheres or lateral lobes and a median or central lobe. Fig. 2.—Motor Nerve-Cells connected by intercellular processes (b, b), and giving origin to outgoing fibers (c, c, c, and a). 4. Multipolar cell containing much pigment around nucleus. Diagrammatic. (Vogt.) The cell-matter lies on the outside, the fibers are within. The external surface of the organ has a foliated appearance, caused by its subdivisions into multitudes of thin plates by numerous fissures. This sub-division allows great increase of cell-matter by numerous fine convolutions, and the matter is further augmented through penetration within of arborescent processes of cell-substance.
The next portion of nerve-matter to be noticed is the cerebrum. This makes up more than four fifths of the contents of the encephalon. The cerebrum is egg-shaped, but flattened on its under side, and lies in the cranium with its small end forward. It is divided into halves or hemispheres by a great longitudinal fissure. These halves, however, are connected by a middle portion of nerve-substance called the corpus callosum. The surface of the cerebrum is molded into numerous convolutions marked off from one another by furrows. The cell-matter of the cerebrum is external, it follows the convolutions, and is from one twelfth to one eighth of an inch in thickness.
The hemispheres of the cerebrum have been divided into lobes called the frontal, parietal, occipital, and temporo-sphenoidal lobes. These divisions are in part arbitrary, while in part they rest upon certain primary fissures, such as the fissure of Sylvius and the fissure of Rolando. It should be borne in mind that these different regions of the cerebrum are not distinct departments physiologically independent. The convolutions are exteriorly connected among themselves and also with convolutions of neighboring lobes. They have, besides, interior connections through bundles of fibers which pass from one convolution to the base of an adjacent convolution. If we remove the encephalic mass and look at it from beneath, we see the medulla as a continuation and enlargement of the spinal-cord; above this we see the pons Varolii, and to right and left the lobes of the cerebellum which lie under the posterior portions of the cerebrum. At the upper part of the pons we see two stems or crura passing to the cerebrum, and serving to join that larger organ with the nerve-matter below.
A little above the crura, and near the center of the mass, we see what is called the Fig. 3.—Division of a very slender nerve fiber, and communication of its branches with a plexus of fibrils in connection with the much branched processes of two nerve-cells. From spinal cord of ox. Magnified 150 diameters. (Gerlach.) optic commissure. This is simply the crossing of the optic nerves on their way to the eyes. Directly below this commissure are two small rounded eminences called the corpora albicantia, while above, on a stem, is the pituitary body. Beyond the optic commissure lie the olfactory bulbs, one on each hemisphere, placed in a slight depression of the surface. If we turn the brain over, its numerous convolutions are seen extending from end to end, from side to side, and also following the lateral surfaces right and left of the great longitudinal fissure.
On removing a horizontal piece from the upper portion of each hemisphere, the cell-matter of the surface will be found to follow the different windings, while the center of each convolution will be seen to be made up of fibers continuous with fiber-matter in the interior of the hemisphere. Should we cut deeper, we would come upon the corpus callosum, the band of connection between the hemispheres; this body is composed almost wholly of fibers passing transversely between the two sides; it makes the hemispheres anatomically and physiologically one. Were we to continue our section straight through the middle line of the callosum, we should reach a lateral chamber in each hemisphere. This chamber contains two rather large bodies called the corpus striatum and the optic thalamus. The stems or crura cerebri previously mentioned pass into these bodies before spreading out through the hemispheres. The striatum is shaped somewhat like a pear; it lies in the chamber with its small end forward, and is composed of alternate layers of cell and fiber matter. The thalamus is ovoid, and presents an almost continuous mass of cell-matter traversed by fibers. The researches of J. Luys, of which a condensed account may be found in Vol. XXXIX of the "International Scientific Series," are most interesting as respects both anatomy and functions of the thalamus. Luys has discovered four isolated ganglia of cell-matter in the thalamus, situated one behind the other. He has also traced connections between these ganglia and certain organs of special sense. Behind and between the thalami are two smaller bodies called the optic lobes. They consist of two rounded eminences, the anterior ones being called the nates, the posterior the testes. The optic tracts, forming the optic commissure previously mentioned, proceed from the nates, the testes being connected by a band of fiber-matter with the cerebellum; commissural fibers join these optic lobes with the thalami.
I have now named the leading portions of the cerebro-spinal system, and have indicated their general connections with one another. They are nothing more nor less than a series of nerve-ganglia connected among themselves by transverse and longitudinal commissures. This system shows us matter in its most highly organized condition; further, this system shows us matter in some positive and necessary relation to consciousness.
The conclusions which we draw respecting the nature of this relation must, as has been said, be determined by a comparison of the
Fig. 4.—Portion of the Trunk of a Nerve, consisting of many smaller cords wrapped up in a common sheath. (Quain, after Sir C. Bell.) A, the nerve; B, a single cord drawn out from the rest. Magnified several diameters.
known functions of the system with the distinctive characteristics of consciousness. I come, therefore, now to consider the functions of the cerebro-spinal system in so far as these are known, and in so far as they may be inferred from recent experiments and pathology.
Functions of the Cerebro-spinal System.—Nerve-matter has, for its general office-work, to bind together the parts of our body. Wherever this matter is divided there is a peculiar division in the organism. This division is peculiar, because it does not affect nutrition or the ordinary organic processes. In a limb whose nerves are severed there is a loss of sensation; there is also a loss of movement; the limb continues to live, but for all limb purposes it might as well be dead. Nerve-matter, therefore, preserves the higher bodily unity.
In examining this general nerve-function we discover the distinctive tasks of fibers and cells. The fibers convey, while the cells originate,
motions. Fibers may convey motions from without inward, or from within outward; in the former case they are called afferent, in the latter efferent. The nerve-arc is composed of an afferent and an efferent fiber and cell matter. The arrangement of the arc is such that the outer end of the afferent fiber terminates at the surface of the body, the inner end at the cell-matter, while the outer end of the efferent fiber terminates in a muscle, its inner end being also in the same cell-matter. Nothing more than this arc is necessary to produce nerve-action. If an impression be made at the surface of the body, the motion there occasioned is carried by the afferent fiber to the cell-substance; through this substance the motion is transferred to the efferent fiber, along which it passes to the muscle causing muscular contraction. Since the cell liberates motion, and, being much more unstable than the fiber, liberates motion freely, it often happens that a slight impression at the surface is followed by a very violent contraction of the muscle.
Our nerve-arc is not a nervous system. We need only one additional element, however, to form such a system, and this is an ascending or "centripetal" fiber which shall proceed to another collection of cell-matter. In the cerebro-spinal system we have the lowest nerve-arc brought into close anatomical relation with the large cell-mass of the cerebral hemispheres, as well as with the lower cell-masses, by ascending and descending fibers.
I am now to indicate the functions of the parts of this system previously described, and first the spinal cord. This organ has two distinct functions—these are transmission of motions and independent nerve activity. As conductor of motions the cord is related to the higher encephalic centers. By transmission of motions from the surface of the body along an afferent fiber to the cell-mass of this cord, and thence to the brain, sensations are made possible. By transmission of motions from the brain along efferent fibers down the antero-lateral columns of the cord to the anterior roots, and thence to muscles supplied by these roots, voluntary movements are made possible. This teaching should be emphasized. We are dependent on the anatomical
Fig. 6.—Under Surface of the Human Brain. (Allen Thomson.) 1, 1. great longitudinal fissure; 2, 2′, 2″, convolutions of under surface of frontal lobe; 3, 3, 3, prolongation to base of the fissure of Sylvius; 4, 4′, 4″, convolutions of the temporal lobe; 5, 5′, occipital lobe; 6. anterior pyramids of medulla; +, posterior extremity of median lobe of cerebellum; 7, 8, 9, 10, lobules of the lateral lobe of the cerebellum. I-IX. Cranial nerves, all but the first more fully seen in the next figure. The ninth nerve of the right side has been removed. X. First cervical nerve.
integrity of the spinal cord and encephalic centers for any direct sensation, knowledge of things affecting nine tenths of our body, and also for any exercise of volition upon these parts of our body. Consciousness and volition, as far as they relate to any direct connection between ourselves and a large part of our physical organism, are entirely conditioned by nerve-matter, and by the special adjustments of this matter found in the spinal cord. It is interesting to remember, in this connection, that the motions which may be the occasion of sensation are carried to the posterior roots of the cord, while those motions which result in movements are carried to the anterior roots of this organ.
These roots are the crescentic-shaped arrangements of cell-substance before described. The functions of the cord are not limited to transmission. Fig. 7.—Under Surface of Cerebral Peduncle, Pons and Medulla, showing Connections of the Cranial Nerves. (Sappey, after Hirschfeld.) 1. infundibulum of pituitary body; 2. part of floor of third ventricle; 3. corpora mamillaria; 4. cerebral peduncles; 5. pons; 6. optic nerves crossing in the middle line so as to form the chiasma; 7. common motor nerves of eyeball; 8. nervus patheticus; 9. trigeminus; 10. external ocular nerve; 11. facial nerve; 12. auditory nerve; 13. nerve of Wrisberg; 14. glossopharyngeal nerve; 15. vagus or pneumogastric; 16. spinal accessory; 17. hypoglossal nerve (cut away on one side). The cord is the source of independent or reflex activities. The peculiarity of these activities is that no consciousness and no volition accompany or occasion them; they are strictly motions. In swallowing food we have an illustration of these reflex activities, and of their close succession to activities that were both conscious and voluntary. Consciously and voluntarily the food is carried to the fauces; at once, the excitation made by the food upon the afferent nerves is carried to the cord and the medulla oblongata; here force is liberated and sent along efferent nerves to the muscular walls of the œsophagus. These walls contract, and the food is passed on into the stomach.
When the cord is broken, those parts of the body which lie below the break will move violently upon irritation, though they can not be moved by any effort of will or be known by any sensation. Many actions, not at first reflex, become so by repetition. Walking is a good example: the movements are learned slowly, and upon numerous efforts; afterward, the work is performed by the centers of the spinal cord, so that walking is really hindered by conscious volition. Dr. Carpenter mentions the case of a shoemaker who was subject to sudden loss of consciousness; at such times he always continued the work he was engaged in when consciousness left him; if walking, he would walk into water or fire; if using his awl, he would continue doing so, frequently to his serious injury.
While this reflex action of the cord may thus take place apart from the brain, the brain exercises a strong inhibitory influence over the action. Some persons, by sheer force of will, can hold their feet still under constant tickling of the soles. The following experiment seems decisive in the matter: A frog is suspended by the head, and his legs are allowed to dip into a vessel of dilute acid. After some time the irritation causes the legs to be removed. The average time is ascertained by frequent trials—then the animal's cord is cut just below the medulla. The time which now intervenes between contact with the acid and withdrawal of the limbs is much shortened, and the action is decidedly more vigorous. Setschenow's experiments (1863) show that this influence of the brain-centers can be greatly augmented by direct irritation of the optic thalami. The rule respecting the reflex activity of the cord would lead us to expect an increase of the activity upon an increase of stimuli. This is true in general, but Wundt has proved that the rule applies to those stimuli only that are carried to the same part of the cord. If an afferent nerve in some other portion of the body should be irritated simultaneously with the cord, reflex action would entirely cease.
We are now to consider certain activities of the cord which are the most remarkable of all its manifestations. I allude to the experiment first performed by Pflüger. This experiment has been frequently repeated and variously interpreted. Pflüger decapitated a frog, and then placed some acetic acid on the animal's thigh. This headless creature immediately wiped off the acid with the bottom of the foot
Fig. 8.—Upper Surface of the Cerebellum. (Sappey. after Hirschfeld.) 1.1. superior "vermiform process" (middle lobe) whose anterior extremity has been pushed backward in order to show the corpora quadrigemina; 2. posterior extremity of the superior and inferior "vermiform processes," and of the median fissure of the cerebellum; 3. great circumferential fissure; 4. great fissure of the upper surface which divides it into two principal segments; 5. posterior of these segments in the form of a crescent; 6, 6, 6, 6, 6, anterior segment, quadrilateral, and composed of five secondary curved segments like the preceding—each of these segments being composed of closely packed "laminæ" of different sizes, separated by fissures of varying depths; 7, 7. sections of the cerebral peduncles; 8. "posterior commissure" of the cerebrum; 9, corpora quadrigemina.
of the same side. Pflüger then removed this foot and again placed acid on the same thigh. The animal at first, as though deceived, endeavored to rub away the acid in the same way as before. This being impossible, the frog soon ceased trying that method, and seemed to be seeking out some other plan. Finally, he made use of the foot which was left, and actually succeeded in removing the acid. Pflüger was so astonished and impressed by his experiment that he declared the spinal cord to be possessed of sensory powers—that is, capable of consciousness. I have stated this experiment in detail because it is the most striking among the many facts which have led to such a conclusion as that of Dr. Hammond in his address at Lehigh University, in October of last year. The address may be found in the November number of "The Popular Science Monthly." Dr. Hammond writes, "Suffice it to say that these experiments all go to establish the fact that the spinal cord, after the complete removal of the brain, has the
Fig. 9.—Inferior Surface of the Cerebellum. (Sappey, after Hirschfeld.) 1, 1, inferior vermiform process; 2, 2, median fissure of the cerebellum; 3, 3, 3, lobes and lobules of the cerebellar hemispheres; 4, "amygdala," or almond-like lobe; 5, lobule of the pneumogastric; 6, pons Varolii; 7, median groove on the same; 8, middle peduncle of the cerebellum; 9, cut surface of medulla; 10, anterior extremity of the great circumferential fissure; 11, anterior border of the upper surface of the cerebellum; 12, motor rout of the trigeminal nerve; 13, sensory root of the same; 14, nerve of the external ocular muscle; 15, facial nerve; 16, nerve of Wrisberg; 17, auditory nerve; 18, glosso-pharyngeal nerve; 19, pneumogastric nerve; 20, spinal accessory nerve; 21, hypoglossal nerve.
power of perception and volition, and that the actions performed are to all intents and purposes as perfect of their kind as they would be were the brain in its place." Though Dr. Hammond does not mention Pflüger's experiment, he cites other instances to the same effect. He has seen "the headless body of the rattlesnake coil itself into a threatening attitude, and, when irritated, strike its bleeding trunk against the offending body." Perrault reports that "a viper whose head had been cut off moved determinedly toward its hole in the wall." Neither these instances, nor the others which Dr. Hammond names in this connection, are as striking as Pflüger's experiment. The noticeable feature in this experiment is the fact that the muscular movements which appear upon irritation of the afferent fibers seem not merely to display a general conformity to ends, but to adjust themselves to changed conditions. Lotze, remembering the involuntary course of acquired movements in man, says, "These actions which point to a consciousness may be simply the back-workings of consciousness upon the mechanism of the reflex organ." Wundt thinks that, "if with Darwin we acknowledge the inheritance of physical dispositions, we may consider these frog activities as properties of the central mechanism wrought out during the entire development of the species, and inherited by the given individual." That either of these opinions is more reasonable than the one of Dr. Hammond, I think there can be little doubt, especially after witnessing the experiment performed by Goltz in 1869. Goltz took two frogs and decapitated one and blindfolded the other; this was done to prevent any voluntary motions that might arise on account of visual impressions. Goltz then placed both animals in a vessel of water and gradually raised the temperature. Both frogs kept quiet until the temperature rose to 25° centigrade; at this point the frog whose brain was uninjured showed signs of discomfort; as the heat increased he tried to escape, and died at 42° C. During this entire time the other frog sat perfectly still, and gave no evidences of distress or pain. But—and here is the significant fact—this same animal, while in the water, made all the reflex defensive efforts when acetic acid was applied to the surface. Aside from these activities, it was still, and died at 50° C. If we admit that the mechanism of the cord possesses the possibility of self-regulation, an admission made by Dr. Hammond in the article from which I have quoted, all these phenomena may be regarded as simple reflex activities. We may accept Dr. Maudsley's judgment that "the reflex activity of the spinal cord is entirely a physical process, which is nowise prevented from taking place because it is not accompanied by consciousness."
Anatomy teaches us to expect complexity of function as we ascend from the cord to the cerebral hemisphere; experiment and pathology confirm our expectation. The medulla oblongata, like the cord, conveys motions to and from the higher centers; further it is the seat of many reflex activities which are indispensable for the organic processes, and further still, it is, in some of these processes, a self-dependent center of innervation. I name the more important activities of the medulla. This organ is the center for respiration. The excitation of this center is brought about in part automatically by the blood. The decrease of oxygen and accumulation of oxidation products in the blood stimulate this part of the medulla, so that respiratory movements may continue after all the afferent nerves connected with the center have been divided. This respiratory mechanism, though truly reflex, is, to a considerable extent, under the control of the will, thus enabling us to articulate for all forms of vocalization.
The medulla is a center of innervation of the heart. Though the heart will beat if completely severed from the cerebro-spinal system, and, in the case of cold-blooded animals, if removed from the body, still its action is decidedly affected by the fibers which unite it with the medulla.
Again, the blood-vessels are brought under the control of the medulla by the vaso-motor center. The vaso-motor nerves pass by the spinal cord to the blood-vessels through the ganglia and fibers of the sympathetic system. These nerves, being constantly active, maintain a tonic contraction of the arterial walls.
The medulla is a center for the movements of chewing and swallowing. This center can be excited in a reflex manner, and by the will, but not automatically. There seems to be good evidence that the medulla is a center for combined or co-ordinated movements of the body. Wundt is of opinion that the collective motor-fibers of the body are brought into closer union with each other in this organ. His opinion rests upon the fact that, as long as the medulla is intact, sensible excitations occasion general movements of the body much more easily than when this organ is destroyed.
The question now arises as to the relation between the medulla and consciousness. I do not think we are justified in supposing that consciousness Fig. 10.—Thalami and Striata. (Sappey, after Hirschleld.) 18, posterior tubercles of the thalami; 19, anterior tubercles of same; 21, veins of the corpus striatum; 23, corpus striatum. appears in connection with this portion of the nervous system. It is a fact that a frog, having simply the cord and medulla, will react not only in the manner already described, but also by movements of the entire body, away from the scarce of trouble; the animal may even utter a cry as if in pain, yet he may be "merely a non-sentient, non-intelligent reflex mechanism." We know that the medulla is the last portion of the nervous system to come under the influence of anæsthetics. Persons submitting to severe surgical operations frequently cry out violently, and as if in intense pain; yet they assure us afterward that they were not conscious of suffering.
It is common to cite, as Dr. Hammond has done in the article before mentioned, those human beings who are born without a cerebellum or cerebrum, but who perform such actions as breathing, sucking, swallowing, and crying. In these cases the spinal cord and medulla oblongata are well developed. Why consciousness should be ascribed to the activities just named it seems difficult to understand, especially when we consider that similar activities can be produced by a machine constructed for the purpose. To say that, "if these activities are not indicative of the existence of mind, we must deny this force to all human beings on their entrance into the world," is a singular declaration—what would be the harm of such denial?
Most human beings on their entrance into the world have the higher cerebral centers, yet they are so soft and undeveloped as to make it doubtful whether consciousness even then appears; certainly it does not except in most elementary form. We have now to inquire respecting the functions of the pons Varolii. Section or irritation of this organ is followed by powerful movements, and much more pronounced signs of pain than any previously manifested. If we cut the anterior portion of one side of the pons, movements will be produced on the opposite side of the body, and the vertebral column will bend toward the side of section. It has been shown that the deeper posterior parts of the pons are made up of transverse fibers connecting the two lobes of the cerebellum, and we find, as we should expect, that injury to one side of this portion of the organ causes the same rolling movements as appear upon one-sided injury to the cerebellum.
If we remove all the encephalic centers above the pons, the animal so treated will maintain his upright position, will give cries quite characteristic of pain, and will bring about conjoined movements of flight. These manifestations disappear completely after removal of the pons, and we have left only those reflex activities already shown to be dependent
Fig. 11.—Longitudinal Section through the Center of the Brain, showing the inner face of Left Cerebral Hemisphere. (Sappey, after Hirschfeld.) 1. spinal cord; 2, pons Varolii; 3, cerebral peduncle; 4. "arbor vitæ;" of cut surface of middle lobe of cerebellum; 5, Svlvian aqueduct; 6, valve of Vieussens; 7, corpora quadrigemina; 8, pineal body: 9, its inferior peduncle; 10, its superior peduncle; 11, middle portion of the great cerebral cleft; 12, upper face of the thalamus; 13, its internal face, forming one of the walls of the middle or third ventricle.
upon the cord and medulla. It seems clear that in the cell-masses of the pons the movements essential for locomotion, for maintenance of upright position, and for expression of pain, are combined. These phenomena have led many physiologists, among them Longet, to consider the pons as a sensorium commune, or the place where the sensations are assembled, and where the movements caused by sensations arise. Other physiologists, among them J. Müller, believe that the pons is the seat of the power of volition. I would reserve my opinion as to the relation between this organ and consciousness until after the functions of other nerve-masses between the pons and cerebral hemispheres have been considered.
The transverse fibers of the pons Varolii unite the lobes of the cerebellum, and we may appropriately consider the functions of this organ before those of the smaller masses within the cerebral hemispheres. There is, perhaps, no subject in nerve-physiology more obscure and difficult than this one of the functions of the cerebellum. The earlier opinion, that this organ is connected with the sexual appetite, has long since been completely disproved. The special difficulty in determining the functions of the cerebellum arises from the disagreement between experiment and pathology, as also from hazard of injury to adjacent nerve-masses. Flourens was the first to investigate the functions of the cerebellum in a strictly inductive manner; his experiments have been repeatedly confirmed, and they must furnish the starting-point for all future inquiry. Flourens says: "I removed the cerebellum of a pigeon in successive slices. During the removal of the first layers there appeared only a weakness and want of harmony in its movements. On removal of the middle layers, the animal exhibited a general agitation, without true convulsions. It made brusque and irregular movements, and continued loath to see and to hear. On removal of the last layers, the animal entirely lost the power of standing, flying, leaping, or walking, which had been gradually affected by the preceding mutilation. Placed on its back, it was unable to rise. Instead of remaining quiet and immovable, like pigeons deprived of their hemispheres, it was in a continual state of restlessness and agitation, but could never make any determinate movement. It could see a threatened blow, and tried to escape, but without success. It made various efforts to recover its station when laid on its back, but utterly failed to do so. Sensation, volition, and intelligence remained, but the co-ordination of movements into regular and determinate movements of progression was entirely lost." There is no doubt that destruction of the cerebellum is frequently followed by striking disorders of equilibrium. Flourens found, however, that these disorders would, in time, be overcome by the animal, even though the lesions were deep. Upon complete destruction of the organ, the disorders were lasting. Weir Mitchell's experiment, quoted by Ferrier, would not confirm this permanency of the disorders. Weir Mitchell states that he destroyed the functional activity of the entire cerebellum in pigeons who, after some months, recovered "so far as to show only feebleness and incapacity for prolonged muscular exertion, but no real inco-ordination or unsteadiness of equilibrium." Repeated experiments have shown a decided difference of result, according to the character and location of the lesions. If these lesions are made symmetrically on both sides, or if the cerebellum be divided in the middle, from the front backward, there is no important disturbance of equilibrium. If, on the other hand, the central lobe be cut in its anterior portion, the animal tends to fall forward; if in its posterior portion, to fall backward. Lesion in one of the lateral lobes is followed by a most violent and rapid whirling of the body, the direction being toward the affected side if the injury extends through the entire lobe, but toward the opposite side if the lesion be partial. Comparing these results of experiments with the teachings of pathology, i. e., with diseases of the cerebellum in man, there is an unexpected disagreement. Changes in one of the lobes may occur without any observable symptoms. It is only when there is a thorough wasting of the lobe that we have marked disturbances, and these are not simply connected with movements, they affect the intelligence as well. This fact has been specially noted by Wundt, who refers to the striking example furnished by Combetti's case of the girl Labrosse. This girl was entirely destitute of cerebellum and pons Varolii and yet was capable of voluntary movements, though showing great muscular weakness and lack of intelligence. Bouillard reports the case of Guérin, whose cerebellum was shown to be almost wholly destroyed, "yet the patient could co-ordinate his movements, even being able to walk." It should be observed that in both these cases there was muscular feebleness, shown by the reeling and tottering motions of the persons diseased. It is customary to mention the ninety-three cases collected Fig. 12.—Human Cerebrum and Cerebellum, showing the relative size of those parts of the Brain. (After Hirschfeld and Léveillé.) by Andral, to show that the cerebellum is not an organ for the co-ordination of movements. Professor Austin Flint, of New York, has made a most careful examination of these cases, and has fully established the fact that none of them, save perhaps two, could possibly be taken to have a bearing on the question. Almost all the cases are complicated with diseases in other brain-masses, or exhibit sufficient disorder of movement to confirm the original position. There is a striking fact, first noted by Purkinje, that leads to what seems the most rational conclusion which our present knowledge will warrant respecting the general function of the cerebellum. Purkinje discovered that a current of electricity passed through the base of the head of a healthy person causes dizziness. It is natural to attribute this result to some action of electricity upon the cerebellum, especially in the light of the experiments already described.
Dizziness is due to some feeling of change in the relation between ourselves and outward objects. This feeling may be produced by an actual change in the objects or a change in ourselves. Illustrations are abundant, such as rapid riding in railroad-trains or violent swinging. It is a fact of importance in this connection, that alcoholism, which so constantly exhibits dizziness, is associated with marked disorder of the cerebellum.
In coming to a conclusion respecting the function of this organ, we find that it can not be directly connected with sensation or volition. The sensations appear pronounced and the movements vigorous after destruction of the cerebellum. The marked feature in all deep cerebellar disorders is the maladjustment of muscular actions to the preservation of equilibrium and to harmonious movements. The consciousness of a normal relation between the person and the external world seems overthrown, the violent activities which ensue being plainly attempts to restore this lost feeling. Movements which are voluntarily initiated must be brought into relation with the position of the body in space. The cerebellum is the organ specially concerned with this work. The regulation of all activities that are willed depends upon this feeling of the accustomed relation between ourselves and objects. The cerebellum, in some unknown way, makes the preservation of this feeling possible.
It should be distinctly borne in mind, however, that this conclusion does not necessitate the further one that the cerebellum is itself a seat of consciousness, not even of this consciousness of normal relation. Consciousness may be entirely conditional upon the activities of the cerebrum, while at the same time this feeling of relation may depend upon the cerebellum. My meaning is that, though consciousness have its sole physical antecedent in the cerebrum, the special form of consciousness now under consideration may be impossible of origination in the cerebrum without the anatomical and physiological integrity of the cerebellum. Before examining the evidence concerning the functions of the cerebrum, a few words should be said with regard to the optic lobes or corpora quadrigemina, the corpora striata, and the optic thalami. The optic lobes are central organs connected with vision. There seems no sufficient reason to doubt the results of experiment as stated by Ferrier: "The more prominent effects of destructive lesions of the optic lobes in the various animals seem to be blindness, paralysis of irido-motor and some oculo-motor reactions, disorders of equilibrium and locomotion, and in frogs, and apparently in other animals, annihilation of certain forms of emotional expression."
If the higher brain-masses be removed, animals show reflex reactions to rays of light, and, more than this, they display other bodily movements evidently due to the influence of light. According to Longet, birds will follow a burning candle with their heads, and frogs that have been startled into movements of flight by irritation of the skin will avoid objects placed in their way.
These lobes are exceedingly sensitive to electrical stimulation, and the results vary as the electrodes are placed on the anterior (nates) or posterior (testes) eminences.
Stimulation of the nates causes wide dilatation of the opposite pupil, the head turns in the direction of the eyes, and the ears are thrown back.
When the testes are stimulated, the same results follow, but with the striking addition that, upon the least touch of the electrodes, cries are produced which change from a brief bark to all kinds of sounds as the stimulation is continued.
The opinion has been held by many, and is explicitly stated by Austin Flint, that "the optic lobes serve as the sole centers presiding over the sense of sight, and not merely as avenues of communication of this sense to the cerebral hemispheres." When Flint gives as "positive"
Fig. 13.—Brain of Gauss, the celebrated Mathematician and Astronomer, upper aspect. (Sharpey, after R. Wagner.) l l, longitudinal fissure; a, a', a", upper, middle, and lower frontal convolutions; A, A, ascending; frontal convolution; r, r, fissure of Rolando; B. B. ascending parietal convolutions; b, b, parietal lobule; b", supra-marginal lobule; c, c', first or upper temporal convolution; p, perpendicular (or parieto-occipital) fissure; d, d', d" upper, middle, and lower occipital convolutions.
proof of his conclusion the statement that "the sense of sight is preserved after complete removal of the cerebrum," he shows how easily it is possible to give, as proof of a conclusion, the conclusion itself. The thing to be determined is, that the actions displayed after removal of the cerebrum are accompanied by any form of consciousness. While it can not be shown that they are not, it is equally impossible to show that they are. The conclusion of Wundt may be the correct one, viz., that these activities are no more designed or conscious than those reflex movements which we know to be produced by the spinal cord.
As to the functions, in detail, of the optic thalami and the corpora striata, hardly anything is known. It will be remembered that these bodies are the rather large masses situated in the chambers of the cerebral hemispheres. There is no reasonable doubt, however, that they are concerned with sensations and motions; the thalami having to do with impressions which are the physical antecedents of sensations, and the striata with the execution of movements. This conclusion is confirmed by pathology. Disease of the striatum is followed by paralysis on the opposite side of the body. Disease of the thalamus, while not always so uniform in its testimony, does still, sometimes, give striking evidence of a sensory significance of the organ. It should be distinctly borne in mind, when speaking of these bodies, that paralysis of sensation and of voluntary motion may be produced by lesions in the cell-matter of the cerebrum apart from any injury to the thalamus and striatum; it should also be remembered that destruction of these basal ganglia breaks the connection between the cell-matter of the cerebrum and the surface of the body, so that the cerebral hemispheres can not perform their functions. It may, therefore, be true, as many maintain, that these organs are not at all directly associated with consciousness, their function being to adjust the connections of sensory and motor fibers with the cerebrum. This general conclusion need not be taken as supporting the fanciful opinions of Luys. Luys believes that in the thalami sensorial impressions "are for the first time condensed, stored up, and elaborated by the individual action of the elements that they disturb in their passage. It is thence that they are launched forth into the different regions of the cortical periphery (cell-matter of the cerebrum) in a new form, intellectualized in some way to serve as exciting materials for the activity of the cells of the cortical substance." According to the same writer, the striata do for our volitions the exact reverse of that which is done by the thalami for our sensory impressions. "It is in the midst of the tissues of the striata that the influence of volition is first received at the moment when it emerges from the psycho-motor centers of the cerebral cortex. There it makes its first halt in its descending evolution and enters into a more intimate relation with the organic substratum destined to produce its external manifestations—in one word, materializes itself." There can be no doubt about the justice of describing this conclusion as fanciful and quite beyond the data.
We have outlined the structure of the cerebro-spinal system, and have stated what may fairly be set down as established concerning the functions of this system up to the cerebral hemispheres. With respect to the presence of consciousness in the parts already examined, it is plain that opinions radically differ. Some maintain that consciousness is not manifested apart from the action of the cerebrum, that all nerve-activities below this organ are reflex, their only distinctions being in the matter of complexity. Others are equally positive that consciousness accompanies all nerve-actions, while still others assert that certain organs below the cerebrum—viz., the pons Varolii, cerebellum, optic lobes—form a sensorium commune where consciousness in some form appears. It is my opinion that this last conclusion has not, as yet, been established or refuted. I regard it as the most rational of the three in the present state of knowledge. If we accept it, we must recognize at the same time a distinction between elementary consciousness and the full consciousness of an intellectual operation. Many facts in every one's experience bear out such a distinction. We are often conscious without knowing the object or occasion of consciousness; being half-aroused, we feel rather than perceive. It is possible, and from the evidence it is even probable, that provision for this rudimentary consciousness is made by the nerve-masses between the medulla and cerebrum.
Whatever conclusion we adopt respecting this matter, the significant fact remains that consciousness is certain to appear in connection with nerve-matter; sooner or later the question of a strictly materialistic interpretation must be faced. After ascertaining the present state of the case with regard to localization of functions in the cerebrum, the induction must be drawn as to the nature of the relation between nerve-matter and consciousness. Grant that this induction shall be more or less a speculation, we need, I think, to remember that all reasoning is speculative, from the nature of the case speculative, and that the only distinction between credulity and reasoning is this, that credulity is both beyond the facts and contrary to the facts, while reasoning is beyond the facts but according to the facts.