23. What do we learn from a comparison of brain weight and intelligence?
24. What is the relation between nervous pathology and mental abnormality?
25. Is phrenology admissible?
26. What view concerning the relation of brain and mind is suggested by the unity of mind?
§ 2. THE NERVOUS SYSTEM
1. _The Elements of the Nervous System_
[Illustration: FIG. 1.--MULTIPOLAR CELL BODY.]
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.
[Illustration: FIG. 2--PYRAMIDAL CELL BODY.
_a_, Nerve fiber with collaterals.]
[Illustration: Fig. 3.--DENDRITES OF A NERVE CELL OF THE CEREBELLUM.]
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).
[Illustration: FIG. 4.--VARIOUS TYPES OF CELL BODIES.
_1_ and _2_, Giant pyramidal cell bodies; _n_, nerve fiber.]
[Illustration: FIG. 5.--LONGITUDINAL SECTION OF A NERVE FIBER WITH STAINED FIBRILS.
_a_, Medullated sheath.]
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.
[Illustration: FIG. 6.--TERMINAL ARBORIZATION OF OPTICAL NERVE FIBERS.]
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´_.
[Illustration: FIG. 7.--DIAGRAM OF NERVOUS ARCHITECTURE: REFLEX ARCHES CONNECTED BY A LOW NERVE CENTER.
(From Psychological Review, 15, 1908.)]
[Illustration: FIG. 8.--DIAGRAM OF NERVOUS ARCHITECTURE: LOWER NERVE CENTERS CONNECTED BY A HIGHER CENTER.
(From Psychological Review, 15, 1908.)]
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.
[Illustration: FIG. 9.--DIAGRAM OF NERVOUS ARCHITECTURE: HIGHER NERVE CENTERS CONNECTED BY A STILL HIGHER CENTER.]
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.
[Illustration: FIG. 10.--FRONTAL SECTION OF THE RIGHT CEREBRAL HEMISPHERE.]
From what has been said thus far it is clear that certain areas of the cortex must be connected with certain groups
[Illustration: FIG. 11.--SECTION OF THE CEREBRAL CORTEX.
Only the cell bodies are stained.]
[Illustration: FIG. 12.--SECTION OF THE CEREBRAL CORTEX.
Only the fibers are stained.]
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.
[Illustration: FIG. 13.--LOCALIZATION OF PERIPHERAL FUNCTIONS IN THE CEREBRAL CORTEX.]
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.
