As we all know, the processes of our mental life stand in the closest relationship with the functions of the nervous system, especially with the functions of its highest organ, the brain. Local anemia, that is, a lack of blood in the brain, causes fainting, a cessation of consciousness; on the other hand, during mental work the blood pressure in the brain is higher than usual and metabolism is increased. Narcotic or poisonous drugs, as alcohol, caffein, and morphine, which influence mental activity, do this by means of their effect on the nervous system. Aside from such experiences, there are two special groups of facts upon which our knowledge of this relationship is based.

First the dependence of mental development on the development of the nervous system. This is most conspicuous when man and animals are compared. It is somewhat obscured, however, by the relation of the size of the brain to the size of the animal. The larger animal has as a rule the larger brain. Therefore the brain of man can be compared only with the brain of such animals as are of nearly the same size. When such a comparison is made, man is found to be no less superior in nervous organization than in intelligence. His brain is about three times as heavy, absolutely and relatively, as that of the animals most nearly approaching him, the anthropoid apes; eight to ten times as heavy as the brain of the most intelligent animals lower down in the scale, for instance large dogs. Similar relations between brain weight and intelligence are found in the human race itself. Of course, we cannot expect that this relation will always be found in a comparison of only two individuals. The conditions are too complex for such a regularity to exist; but it is easily demonstrated when averages of groups of intelligent and unintelligent men are compared. We do not expect, either, that in every individual case physical strength is exactly proportional to the weight of the muscles, although no one doubts that strength depends on the weight of the muscles.

The second of the facts upon which our knowledge of the relationship between mental life and nervous function is based, consists in the parallel effects of disturbances of their normal condition. Diseases or injuries of the brain are, as a rule, accompanied by disturbances of the mental life. On the other hand, mental disturbances can often be traced to lesions or structural modifications in the brain. This cannot be done in every case; but the actual connection is none the less certain. It is often very difficult to decide whether or not any mental abnormality exists. Expert psychiatrists have for weeks at a time observed men suspected of mental disease without being able to pronounce judgment. Equally difficult is the discovery of material changes in the brain and its elements. Much progress has been made in recent times in this respect; but it is still far from easy to recognize the more delicate changes in nervous structure resulting from disease. Certain abnormalities may never become directly visible although they involve disturbances of function, for instance, abnormalities in the nutrition of the nervous elements or changes in their normal sensitivity. No wonder, then, that for many mental diseases, as hysteria, corresponding material lesions are not yet known. But the correctness of our thesis is so strongly secured by the enormous number of cases in which it has been demonstrated, that no one doubts that it applies also to those cases in which, often for good reasons, its demonstration has thus far been impossible.

Of much importance is the particular form of this relationship between brain function and mental life. Popular thought attributes the chief classes of total mental activity to special parts of the brain. Judgment is thought to have its seat behind the thinker’s high forehead. The occipital part of the brain is, according to the medieval philosophers, the organ of memory. And so Gall’s phrenology met with ready acceptance from the public at large, which was delighted to learn that musical ability, mathematical talent, religious sentiment, egotism and altruism, and many other character traits had their special organs in the brain. But anatomists and physiologists have not been able to admit the plausibility of this doctrine.

Yet popular thought has, on the other hand, always emphasized the unity of mind. Those who regard its unity as the chief characteristic of mind have for centuries sought for the single point in the brain where the mind can be said to have its seat. If it were distributed all through the brain, would it not be possible to cut the mind into pieces by simply cutting the brain?

That both these views of the relation between brain and mind are inadmissible has become certain. Since about forty years ago the truth in this matter has been known. But to understand it clearly it is necessary first to familiarize ourselves with the construction of the nervous system.

§ 2. The Nervous System

1. The Elements of the Nervous System

The number of elements making up the nervous system is estimated at about four thousand millions. It will help us to comprehend the significance of this number if we understand that a man’s life devoted to nothing but counting them would be too short to accomplish this task, for a hundred years contain little more than three thousand million seconds. These elements are stringlike bodies, so thin that they are invisible to the naked eye. They are generally called neurons. Within them different parts are to be distinguished. The part which is most important for the neuron’s life is a spherical, bobbin-shaped, pyramidal, or starlike body, called the ganglion cell or cell body, located usually near one of the ends of the long fiber of the neuron, but sometimes nearer the middle of the fiber. The length of the fiber varies from a fraction of an inch to several feet. The fiber may be compared with a telephone wire, inasmuch as its function consists in carrying a peculiar kind of excitatory process.

At both ends of the neuron are usually found treelike branches. When the cell body is located near one of the ends of the fiber, many of these branches take their origin from the cell body and give it the pyramidal or starlike appearance illustrated by figures 1, 2, and 4. These branches are called dendrites, from the Greek word for tree, dendron. How wonderfully complicated the branching of a neuron may be is illustrated by figure 3. In addition to the dendrites a neuron possesses another kind of branches, resembling in character the tributaries of a large river, entering into it at any point of its course. These are called collaterals (lowest part of figure 2).

The ganglion cells have a varying internal structure, which may be made visible to the eye when the cells have been stained by the use of different chemicals. They are found to contain small corpuscles with a network of minute fibrils between them, as shown in figures 1 and 4. The nerve fibers, too, in spite of being only ¹/₄₀ to ¹/₅₀₀ mm. thick, permit us to distinguish smaller parts (fig. 5). The core consists of a bundle of delicate, semi-fluid, parallel fibrils, the axis-cylinder. This is surrounded generally by a fatty, marrow-like sheath, and in the peripheral parts of the system this sheath is again inclosed in a membrane. Certain fibers attain a considerable length, for example, those which end in the fingers and toes, having their origin in the spinal region of the body.

The treelike branches of the main fiber and of the collaterals, if far away from the cell body, are sometimes called the terminal arborization, from the Latin word for tree, arbor (fig. 6). The treelike branching has most probably a functional significance of great importance. It enables the endings of different neurons to come into close enough contact to make it possible for the nervous processes to pass over from one neuron into another neuron, without destroying the individuality, the relative independence of each neuron.

Wherever large masses of neurons are accumulated, the location of the ganglion cells can be found directly by the naked eye. The fibers are colorless and somewhat transparent. Where they are massed together, the whole looks whitish, as is the case with snow crystals, or foam. The ganglion cells, however, contain a dark pigment, and where many of them are present among the fibers, the whole mass looks reddish gray. Accordingly one speaks of white matter and gray matter in the nervous system.

The nature of the excitatory process for the carriage of which the neurons exist is still unknown. It is certain, however, that this process is not an electrical phenomenon. Electrical changes accompany the nervous process and enable us to recognize its presence and even to measure it; but they are not identical with the nervous process. Probably it is a kind of chemical process, perhaps analogous to the migration of ions in the electrolyte of a galvanic element, the lost energy being restored by the organism. Two facts are especially noteworthy. The velocity of propagation has been found to be about 60 meters per second in the human nervous system. In the lowest animals propagation is often considerably slower. It is clear, therefore, that it is an altogether different magnitude from the velocities found in light, electricity, or even sound.

A second fact is the summation of weak stimulations. The second one produces a stronger effect than the first, the third again a stronger effect, and so on. It also happens that a number of successive stimuli produce a noticeable effect, whereas one of these stimuli alone, on account of its weakness, would produce none. On the other hand, if strong stimuli succeed one another, the effect becomes less and less conspicuous. The neurons are fatigued, as we say, and require time for recuperation.

2. The Architecture of the Nervous System

The elements of the nervous system just described are combined into one structure according to a surprisingly simple plan, in spite of its seeming complexity. This apparent complexity results chiefly from the enormous number of elements entering into the combination. The purpose of the nervous architecture may be briefly described thus: The conductivity of the nervous tissue is employed to bring all the sensory points of the living organism into close connection with all the motor points, thus making a body capable of unitary action out of a mere accumulation of organs, each of which serves its specific end. Walking along and meeting an obstacle, I must be able first to look about and find a way of pushing it aside or climbing over it, and then to push or climb. This is impossible unless my eyes are connected with the muscles of the head, the arms, the legs. Perhaps I am inattentive, or it is dark, so that I run against the obstacle with my feet or my body. In this case it is necessary that the sensory points of my skin be connected with all those muscles. Hearing a call, I must be able to turn my head so that I may hear more distinctly the sound I am expected to perceive; but I must also be able to move my tongue and the rest of my vocal organs in order to answer, or, as the case may require, my arms and legs in order to defend and protect myself. Thus the ear and all other sensory points of the body must be closely connected with all the motor points.

It is plain, then, that the simplest kind of nervous system must consist of three kinds of neurons: sensory (often called afferent), motor (often called efferent), and connecting neurons. To improve the working of such a system, the afferent and the efferent neurons, and especially the connecting (associating) paths, are developed by the introduction of additional neurons, serving to cross-connect the primary chains of neurons. Figure 7 illustrates the architecture of an exceedingly simple nervous system of the most rudimentary kind.

A perfection of the system is brought about by a superstructure built on essentially the same plan. Figure 8 is a diagram illustrating this. The points S´ and M´ correspond to the points of the same names in figure 7. But several systems (three in the diagram) like that of figure 7 have been combined by connecting neurons in exactly the same manner in which the combination was effected in figure 7. In this higher system (nerve center, we should call it) the points S´´´ and M´´ have a significance comparable to that of S´ and M´.

Several of these larger systems (three in the diagram) are combined again by means of connecting neurons in exactly the same manner as before. This is illustrated by figure 9. The points S´´´ and M´´´ have a significance like that of S´ and M´, S´´´ being nearer to sensory points of the body than to motor points, M´´´ being nearer to motor points. This system of connecting neurons represents again what we may call a higher nerve center—higher still than those which are combined in it.

Thus we may conceive any number of systems, one still higher than the other. And we may understand how it is possible that simpler mental functions may enter into a combination, forming a unitary new function, without completely losing their individuality as functions of a lower order; for combinations of simple functions represented by direct connections into complex functions are brought about only by mediation of higher connecting neurons which represent the less direct connections of sensory and motor points. The most manifold associations are made possible. A practically inexhaustible number of different adaptations is structurally prepared, so that the most complicated circumstances and situations find the organism capable of meeting them in a useful reaction. This type of nervous system is the property of the highest animals and of man. The lower type of nervous system is represented by the reflex arches of the so-called spinal and subcortical centers. The higher type is represented by the cerebrum and cerebellum, which during a process of evolution covering hundreds of thousands of years have gradually been developed to serve as the highest centers of the nervous system.

3. The Anatomy of the Nervous System

The most prominent part of the nervous system is that inclosed within the skull and the vertebral column. The spinal cord runs all through this column up to the skull. Entering into the skull, it thickens and forms what is called the bulb (medulla oblongata). It then divides into several bodies, which are referred to as the subcortical centers, because they are located below the cortex, which is the surface layer of the cerebrum, or large brain. These subcortical centers contain the central ends of neurons which are links of chains of afferent neurons coming from the higher sense organs and from the sensory points of the skin and the internal organs. Chains of efferent neurons, on the other hand, take their origin in the subcortical centers, reaching at their peripheral ends the motor points of the body, that is, the muscle fibers of our skeletal muscles and of the muscle tissues contained in the alimentary canal and the other internal organs.

Above and partly surrounding the subcortical centers are the large brain and the cerebellum or small brain. The ganglion cells of the neurons contained in the cerebrum and cerebellum are all located near the surface or cortex. There seems to be a peculiar advantage—not yet perfectly understood—in having the gray matter spread out over the surface of the cerebrum and cerebellum in as thin a layer as possible. To this end the surface of the cerebrum is much increased by the formation of large folds, separated by deep fissures (see figure 10). In the cerebellum the folds are more numerous and exceedingly fine, and they do not have the appearance of being the product of fissuration. The surface of the cerebrum is estimated to be equal to a square with a side eighteen inches long. Without the fissures the surface would be only about one third of this. The mixture of ganglion cells and fibers making up the gray matter of the brain is illustrated in figures 11 and 12. Both are sections of the cortex of the cerebrum. In figure 11 the cell bodies alone are stained and thus made visible; in figure 12 the fibers alone are stained.

From what has been said thus far it is clear that certain areas of the cortex must be connected with certain groups

of sensory points or motor points of the body much more directly than with others. This is confirmed by histological, pathological, and experimental investigations. For the eyes and the ears, for the muscles of arms and legs, hands and feet, even the several fingers and toes, the corresponding areas of the cortex—that is, the areas with which there is direct connection—are definitely known. Figure 13 conveys an idea of the relation between certain parts of the brain and the sensory and motor organs of the body.

4. The Nervous System and Consciousness

We have already touched on the question as to the relation between the nervous system and consciousness. It is evident that no single point of the nervous system can be regarded as the long-searched-for seat of the soul, since no single point is structurally or functionally distinguished from all others. But it does not follow that mental functions are localized in different parts of the brain according to the popular conception of judgment, memory, will, and so on, each depending on a special part of the brain. There is no more truth in the similar assertions of phrenology. Localization of function in this sense is impossible. Judgment is not a mental function which can be separated from memory and attention. No more separable from each other are such functions as religious sentiment, filial love, self-consciousness. The sensational, ideational, and affective elements of these functions are to a considerable extent the same.

Localization of mental functions really means this:—Since there is a division of labor among the sensory and motor organs of the body, and since each of these organs is most directly connected with certain areas of the cortex and much less directly with the other areas, it is to be expected that certain states of consciousness will occur only when certain areas of the cortex are functioning. It is but natural that the province of the cortex most directly connected with the eyes serves vision, including both visual perception and visual imagination; that the province of the cortex most directly connected with the ears serves audition. Who would expect anything else? In the same sense, the sensations of touch, of taste, and so on, are localized in the brain. The same rule holds good for movements. When our limbs move in consequence of some thought concerning them, the areas of the cortex which are most closely connected with them must function, while other areas may remain inactive. Activity of our vocal organs, in the service of our mind, can occur only by the influence of that province of the cortex which is most directly connected with the muscles of the vocal organs. But how varied are the thoughts which may bring about action of the vocal organs! On the other hand, how diversified may be the movements by which a mother may react upon the crying of her child! In either case it may be right to say that our mind is localized in the brain as a whole—not, of course, equally in every infinitesimal particle, but distributed through the brain in a manner comparable to the distribution of the roots and branches of a tree.

§ 3. Explanation of the Functional Relation between Brain and Mind

How the functional relation between the mind and the nervous system should be explained, is a question discussed for centuries and variously answered. But all the answers are essentially either the one or the other of these two: (1) Either the brain is a tool of the mind, or (2) it is an objectified conception of the mind itself.

1. The Brain a Tool of the Mind

Popular thought, supported by desires common to all human beings, readily accepts the view that mind is essentially different from matter, that its laws are in every respect different from the laws of material nature, and that the brain, being a part of the material nature, is simply the special tool used by the mind in its intercourse with nature. Consider what a contrast seems to exist between logical certainty and the mere probability derived from more or less deceptive sense impressions, between voluntary attention and sensual desire, between religious inspiration and ordinary perception, artistic creation and everyday work. Nevertheless, these highest as well as the lowest activities of the mind need a tool with which they can get into communication with the world; and this tool, says popular thought, is the brain. By means of this tool the mind can take possession of the world and shape it at will. This explanation of the functional relation between the mind and the nervous system agrees well with the facts above discussed concerning brain weight and intelligence, and nervous pathology and mental abnormality. That the magnitude, the architecture, the normal condition of a tool have an influence on the task performed, is plain enough. Many a piece of music can be played on a large organ having a great variety of stops, whereas its performance on a small instrument would be impossible. Raffael might have deserved the name of a great painter if born without arms, but the world would never have known it.

The facts of localization of function, however, do not agree so well with this tool conception of the brain, which always leads us back again to the theory that the mind takes hold of its tool at a single point. If the mind can suffer or produce this change only here, that change only there, it is difficult to see why we should regard it as an altogether separate entity. Some have pointed out, as an analogy, that truth too is everywhere, and because of its absolute unity, everywhere in its totality, without being bound to space and time. I must doubt, however, if truth is present where such analogies are worked out, for nothing can be less clear than the assertion that truth has unity. Mind is not everywhere in its totality, neither in the brain nor in the whole world. It is partly here, partly there; as seeing mind it is in the occipital convolutions of the brain, as hearing mind in the temporal convolutions. Thus we are forced, if we regard the brain as the mind’s tool, to regard the mind as an entity possessing spatial form. If we reject this conclusion, we must also reject the premise that the brain is the mind’s tool.

There are two other difficulties of very considerable importance. One of them is compliance with the principle of the conservation of energy. If mind is an entity independent of the brain, if the brain is a tool which mind can use arbitrarily, without having to obey the laws of the material world, there would be a serious break in the continuity of natural law, and the principle of the conservation of energy would suffer an exception.

Until recently it was, not probable, but at least possible, that this principle of the conservation of energy was not strictly correct when applied to conscious beings, especially to man. But in recent years direct experiment has proved that it applies to the dog, and even to man. In an animal performing no gross muscular work the energy supplied by the food is completely transformed into heat, which is absorbed by the animal’s surroundings. Rubner has found as the result of very exact measurements that the heat produced by an animal during several weeks is within one half of one per cent (that is, within the probable error) equal to the quantity of chemical energy received from the food. One might think that it would be rash to apply conclusions reached by experimenting on a dog to man, whose mental life stands on a much higher level. But even this objection has been removed by Atwater. He performed similar experiments on five educated persons, varying the conditions of mental and muscular activity or relative rest. The result is the same. Taking the total result, there is absolute equality between the energy supplied and the energy given out; in the human organism, mind has thus been proved to be subject to the laws of the natural world.

The second difficulty spoken of consists in the fact that, accepting the view which regards the brain as the mind’s tool, we cannot well avoid regarding the mind as a kind of ghost or demon, similar to the demons with which the imagination of primitive peoples populates the universe—gaseous and usually invisible men, women, giants, or dwarfs. Mankind has always felt strongly inclined to believe in the existence of such demons, and is still fond of making them the subjects of fairy tales and similar stories. But the more mature experience of the last centuries of human history has eliminated them from our theories of the actual world and assigned them their proper places in tales and mythology. Winter and summer, rain and sunshine, even the organic processes in the heart or the spinal cord are understood only by excluding from the explanation the assumption of such demons. The same is by analogy true for the processes in the brain, for the brain is not likely to be an exception to the rule. It is more difficult, of course, to determine directly whether such a demon exerts his influence in the inaccessible cavity of the skull than it is on the street or even in a haunted house. But no assertion is entitled to be regarded as true merely because we cannot go to the place in question and observe that it is false. Why not assert that heaven is located on the back side of the moon and hell in the center of the sun, merely because no one can see with his own eyes that they are not there? We must make only those assumptions which, considered from all points of view, have a high degree of probability, not those which flatter our vanity or appeal to us as the fashionable belief of the time. Now, it does not seem probable that our brain is the residence of a separable demon, no matter whether we attribute to him the power of changing at will the total amount of energy contained in our body, or conceive his activity, as some psychologists do, as a new form of energy added to the mechanical, thermal, electric, chemical, and so on,—requiring only an additional transformation of energy and not breaking down the principle of its conservation.

2. The Brain an Objectified Conception of the Mind

If we cannot regard the brain and the mind as two independent entities, scarcely any other conception of them is possible except as a single entity of which we may obtain knowledge in two ways, an objective and a subjective way. Mind knows itself directly, without mediation of any kind, as a complex of sense impressions, thoughts, feelings, wishes, ideals, and endeavors, non-spatial, incessantly changing, yet to some extent also permanent. But mind may also be known by other minds through all kinds of mediations, visual, tactual, and other sense organs, microscopes and other instruments. When thus known by other minds, mind appears as something spatial, soft, made up of convolutions, wonderfully built out of millions of elements, that is, as brain, as nervous system. By mind and brain we mean the same entity, viewed now in the aspect in which mind knows itself, now in the aspect in which it is known by other minds.

Suppose a person is asked a question and after some hesitation replies. In so far as this act is seen, heard, and otherwise perceived (or imagined as seen, heard, or otherwise perceived), it is a chain of physical, chemical, neurological, etc., processes, of material processes as we may say. But that part of the chain of material processes which occurs in the nervous system may not only be known by others, but may know itself directly, as a transformation of perceptual consciousness into thought, feeling, willing. The links of these two chains of material processes in the brain and of mental states should not be conceived as intermixed and thus forming one new chain, but rather as running parallel—still better as being link for link identical. The illusion that one of these chains brings forth the other is caused by the fortuitous circumstance that they do not both become conscious at once. He who thinks and feels cannot at the same time experience through his sense organs the nervous processes as which these thoughts and feelings are objectively perceptible. He who observes nervous processes cannot at the same time have the thoughts and feelings as which these processes know themselves. Those objective processes, however, which go on outside of the nervous system, in particular those outside of the experiencing organism, in the external world, precede or follow mental states as causes generally precede their effects and effects follow their causes. There is no objection to speaking of a causal relation between material processes of this kind and mental states.

Whatever explanation of the functional relation between brain and mind a person may accept, he need not constantly be on his guard lest he be inconsistent. We speak of the rising and setting sun without meaning that the earth is the center of the universe and that the sun moves around it. So we may also continue to speak quite generally of the material world as influencing our mind, and of the mind as bringing about changes in the material world.

Our view of the relation between body and mind leads to the further conclusion that, as our body may be distinguished from its parts without having existence separate from its parts, so our mind may be distinguished from the several states of consciousness without having existence separate from them. Mind is the concept of the totality of mental functions. As self-preservation is the chief end of all bodily function, so self-preservation is the chief end of mental life.