Chapter 54

MIND AND BODY

=Relation of Mind to Bodily Organism.=--Notwithstanding the antithesis which has been affirmed to exist between mind and matter, yet a very close relation exists between mind and the material organism known as the body. There are many ways in which this intimate connection manifests itself. Mental excitement is always accompanied with agitation of the body and a disturbance of such bodily processes as breathing, the beating of the heart, digestion, etc. Such mental processes as seeing, hearing, tasting, etc., are found also to depend upon the use of a bodily organ, as the eye, the ear, the tongue, without which it is quite impossible for the mind to come into relation with outside things. Moreover, disease or injury, especially to the organs of sense or to the brain, weakens or destroys mental power. The size of the brain, also, is found to bear a certain relation to mental capacity; the weight of the average brain being about 48 ounces, while the brain of an idiot often weighs only from 20 to 30 ounces.

THE NERVOUS SYSTEM

=Divisions of Nervous System.=--This intimate connection between mind and body is provided for through the existence of that part of the bodily organism known as the nervous system, and it is this part, together with its associated organs of sense, that chiefly interests the student of psychology. A study of the character and functions of the various parts of the nervous system, and of the nervous substance of which these parts are composed, belongs to physiology rather than to psychology. As the student-teacher is given a general knowledge of the structure of the nervous system in his study of physiology, a brief description will suffice for the present purpose. The nervous system consists of two parts, (1) the central part, or cerebro-spinal centre, and (2) an outer part--the spinal nerves. The central part, or cerebro-spinal centre, includes the spinal cord, passing upward through the vertebrae of the spinal column and the brain. The brain consists of three parts: The cerebrum, or great brain, consisting of two hemispheres, which, though connected, are divided in great part by a longitudinal fissure; the cerebellum, or little brain; and the medulla oblongata, or bulb. The spinal nerves consist of thirty-one pairs, which branch out from the spinal cord. Each pair of nerves contains a right and left member, distributed to the right and the left side of the body respectively. These nerves are of two kinds, sensory, or afferent, (in-carrying) nerves, which carry inward impressions from the outside world, and motor, or efferent, (out-carrying) nerves, which convey impulses outward to the muscles and cause them to contract. There are also twelve pairs of nerves connected with the eye, ear, nose, tongue, and face, which, instead of projecting from the spinal cord, proceed at once from the brain through openings in the cranium. These are, therefore, known as cerebral nerves. In their general character, however, they do not differ from the projection fibres.

=Nervous Substance.=--Nervous substance is divided into two kinds--grey, or cellular, substance and white, or fibrous, substance. The greater part of the grey matter is situated as a layer on the outside of the cerebrum, or great brain, where it forms a rind from one twelfth to one eighth of an inch in thickness, known as the cortex. It is also found on the surface of the cerebellum. Diffuse masses of grey matter are likewise met in the other parts of the brain, and extending downward through the centre of the spinal cord. The function of the grey matter is to form centres to which the nerve fibres tend and carry in stimulations, or from which they commence and carry out impulses.

=The Neuron.=--The centres of grey matter are composed of aggregations, or masses, of very small nerve cells called neurons. A neuron may range from 1/300 to 1/3000 of an inch in diameter, and there are several thousand millions of these cells in the nervous system. A developed neuron consists of a cell body with numerous prolongations in the form of white, thread-like fibres. The neuron with its outgoing fibres is the unit of the nervous system. Neurons are supposed to be of three classes, sensory to receive stimulations, motor to send out impulses to the muscles, and association to connect sensory and motor centres.

These neurons, as already noted, are collected into centres, and the outgoing fibres give connection to the cells, the number of connections for each neuron depending upon its outgoing fibres. Some of these connections are already established within the system at birth, while others, as we shall see more fully later, are formed whenever the organism is brought into action in our thinking and doing. To speak of such connections being formed between nerve centres by means of their outgoing fibres does not necessarily mean a direct connection, but may imply only that the fibres of one cell approach nearly enough to those of another to admit of a nervous impulse passing from the one cell to the other. This is often spoken of as the establishment of a path between the centres.

=The Nerve Fibres.=--The nerve fibres which transmit impressions to and from the centres of grey matter average about 1/6000 of an inch in thickness, but are often of great length, some extending perhaps half the length of the body. Large numbers of these fibres unite into a sheath or single nerve. It is estimated that the number of fibres in a single nerve number in most cases several thousand, those in the nerve of sight being estimated at about one hundred thousand. The fibres in the white substance of the brain are estimated at several hundred million.

=Classes of Fibres.=--These fibres are supposed to be of four classes, as follows:

1. _Sensory Cerebral and Spinal Fibres_

These have already been referred to as spreading outward from the brain and spinal cord to different parts of the body. Their office is, therefore, to carry inward to the centres of grey matter impressions received from the outside world, thus setting up a connection between the various senses and the cortex of the brain.

2. _Motor Cerebral and Spinal Fibres_

These fibres connect the centres of grey matter directly with the muscles, and thus provide a means of communication between these muscles and the cortex of the brain.

3. _Association Fibres_

These connect one part of the cortex with another within the same hemisphere.

4. _Commissural Fibres_

These connect corresponding centres of the two hemispheres of the cerebrum.

=Function of Parts.=--Because the various cells are thus brought into relation, the whole nervous system combines into a single organism, which is able to receive impressions and provides conditions for the mind to interpret these impressions and, if necessary, react thereon. When, for instance, a stimulus is received by an end organ (the eye), it will be transmitted by a sensory nerve directly inward to a sensory centre, or cell, in the cortex of the brain. In such a case it may be interpreted by the mind and a line of action decided upon. Then by means of associating cells and fibres a motor centre may be stimulated and an impulse transmitted along an outgoing motor nerve to a muscle, whereupon the necessary motor reaction will take place. A pupil may, for instance, receive the impression of a word through the ear or through the eye and thereupon make a motor response by writing the word. The arrows in the accompanying figure indicate the course of the stimulus and the response in such cases.

THE CORTEX

=Cortex the Seat of Consciousness.=--Experiments in connection with the different nerve cords and centres have demonstrated that intelligent consciousness depends upon the nerve centres situated in the cortex of the cerebrum. For instance, a sensory impulse may be carried inward to the cells of the spinal cord and upward to the cerebellum without any resulting consciousness. When, however, the stimulus reaches a higher centre in the cortex of the brain, the mind becomes conscious, or interprets the impression, and any resulting action will be controlled by consciousness, through impulses given to the motor nerves. It is for this reason that the cortex is called the seat of consciousness, and that mind is said to reside in the brain.

=Localization of Function.=--In addition, however, to placing the seat of consciousness in the cortex of the brain, psychologists also claim that different parts of the cortex are involved in different types of conscious activity. Sensations of sight, for instance, involve certain centres in the cortex, sensations of sound other centres, the movements of the organs of speech still other centres. Some go so far as to claim that each one of the higher intellectual processes, as memory, imagination, judgment, reasoning, love, anger, etc., involves neural activity in its own special section of the cortex. There seems no good evidence, however, to support this view. The fact seems rather that in all these higher processes, quite numerous centres of the cortex may be involved. The following figure indicates the main conclusions of the psychologists in reference to the localization of certain important functions in distinct areas of the cortex.

=Nature of Reflex Action.=--While a lower nerve centre is not a seat for purposeful consciousness, these centres may, in addition to serving as transmission points for cortical messages, perform a special function by immediately receiving sensory impressions and transmitting motor impulses. A person, for instance, whose mind is occupied with a problem, may move a limb to relieve a cramp, wink the eye, etc., without any conscious control of the action. In such a case the sensory impression was reported to a lower sensory centre, directly carried to a lower motor centre, and the motor impulse given to perform the movement. In the same way, after one has acquired the habit of walking, although it usually requires conscious effort to initiate the movements, yet the person may continue walking in an almost unconscious manner, his mind being fully occupied with other matters. Here, also, the complex actions involved in walking are controlled and regulated by lower centres situated in the cerebellum. In like manner a person will unconsciously close the eyelid under the stimulus of strong light. Here the impression caused by the light stimulus, upon reaching the medulla along an afferent nerve, is deflected to a motor nerve and, without any conscious control of the movements, the muscles of the eyelid receive the necessary impulse to close. Actions which are thus directed from a lower centre without conscious control, are usually spoken of as reflex acts. Acts directed by consciousness are, on the other hand, known as voluntary acts. The difference in the working of the nervous mechanism in consciously controlled and in reflex action may be illustrated by means of the accompanying figures.

The heavy lines in Figure 1 on the opposite page show that the sensory-motor arc is made through the cortex, and that the mind is, therefore, conscious both of the sense stimulus and also of the resulting action. Figure 2 shows the same arc through a lower centre, in which case the mind is not directly attending to the impression or the resulting action.

=Function of Consciousness.=--The facts set forth above serve further to illustrate the purposeful character of consciousness as man interprets and adjusts himself to his surroundings. So long, for instance, as the individual walks onward without disturbance, his mind is free to dwell upon other matters, cortical activity not being necessary to control the process of walking. If, however, he steps upon anything which perhaps threatens him with a fall, the rhythmic interplay between sensory and motor activity going on in the lower centres is at once disturbed, and a message is flashed along the sensory nerve to the higher, or cortical, centres. This at once arouses consciousness, and the disturbing factor becomes an object of attention. Consciousness thus appears as a means of adaptation to the new and varying conditions with which the organism is confronted.

CHARACTERISTICS OF NERVOUS MATTER

=A. Plasticity.=--One striking characteristic of nervous matter is its plasticity. The nature of the connections within the nervous system have already been referred to. Mention has also been made of the fact that numerous connections are established within the nervous system as a result of movements taking place within the organism during life. In other words, the movements within the nervous system which accompany stimulations and responses bring about changes in the structure of the organism. The cause for these changes seems to be that the neurons which chance to work together during any experience form connections with one another by means of their outgrowing fibres. By this means, traces of past experiences are in a sense stored up within the organism, and it is for this reason that our experiences are said to be recorded within the nervous system.

=B. Retentiveness.=--A second characteristic of nervous matter is its retentive power. In other words, the modifications which accompany any experience, besides taking on the permanent character referred to above, pre-dispose the system to transmit impulses again through the same centres. Moreover, with each repetition of the nervous activity, there develops a still greater tendency for the movements to re-establish themselves. This power possessed by nervous tissue to establish certain modes of action carries with it also an increase in the ease and accuracy with which the movements are performed. For example, the impressions and impulses involved in the first attempts of the child to control the clasping of an object, are performed with effort and in an ineffective manner. The cause for this seems to be largely the absence of proper connections between the centres involved, as referred to above. This absence causes a certain resistance within the system to the nervous movements. When, however, the various centres involved in the movements establish the proper connections with one another, the act will be performed in a much more effective and easy manner. From this it is evident that the nervous system, as the result of former experiences, always retains a certain potential, or power, to repeat the act with greater ease, and thus improve conduct, or behaviour. This property of nervous matter will hereafter be referred to as its power of retention.

=C. Energy.=--Another quality of nervous matter is its energy. By this is meant that the cells are endowed with a certain potential, or power, which enables them to transmit impressions and impulses and overcome any resistance offered. Different explanations are given as to the nature of this energy, or force, with which nervous matter is endowed, but any study of these theories is unnecessary here.

=D. Resistance.=--A fourth characteristic to be noted regarding nervous matter is that a nervous impulse, or current, as it is transmitted through the system, encounters _resistance_, or consumes an amount of nervous energy. Moreover, when the nervous current, whether sensory or motor, involves the establishment of new connections between cells, as when one first learns combinations of numbers or the movements involved in forming a new letter, a relatively greater amount of resistance is met or, in other words, a greater amount of nervous energy is expended. On the other hand, when an impulse has been transmitted a number of times through a given arc, the resistance is greatly lessened, or less energy is expended; as indicated by the ease with which an habitual act is performed.

=Education and Nervous Energy.=--It is evident from the foregoing, that the forming of new ideas or of new modes of action tends to use up a large share of nervous energy. For this reason, the learning of new and difficult things should not be undertaken when the body is in a tired or exhausted condition; for the resistance which must be overcome, and the changes which must take place in the nervous tissue during the learning process, are not likely to be effectively accomplished under such conditions. Moreover, the energy thus lost must be restored through the blood, and therefore demands proper food, rest, and sleep on the part of the individual. It should be noted further that nervous tissue is more plastic during the early years of life. This renders it imperative, therefore, that knowledge and skill should be gained, as far as possible, during the plastic years. The person who wishes to become a great violinist must acquire skill to finger and handle the bow early in life. The person who desires to become a great linguist, if he allows his early years to pass without acquiring the necessary skill, cannot expect in middle life to train his vocal organs to articulate a number of different languages.

=Cortical Habit.=--In the light of what has been seen regarding the character and function of the nervous system, it will now be possible to understand more fully two important forms of adjustment already referred to. When nervous movements are transmitted to the cortex of the brain, they not only awaken consciousness, or make the individual aware of something, but the present impression also leaves certain permanent effects in the nervous tissue of the cortex itself. Since, however, cortical activity implies consciousness, the retention of such a tendency within the cortical centres will imply, not an habitual act in the ordinary sense, but a tendency on the part of a conscious experience to repeat itself. This at once implies an ability to retain and recall past experiences, or endows the individual with power of memory. Cortical habit, therefore, or the establishment of permanent connections within the cortical centres, with their accompanying dynamic tendency to repeat themselves, will furnish the physiological conditions for a revival of former experience in memory, or will enable the individual to turn the past to the service of the present.

=Physical Habits.=--The basis for the formation of physical habits appears also in this retentive power of nervous tissue. When the young boy, for instance, first mounts his new bicycle, he is unable, except with the most attentive effort and in a most laboured and awkward manner, either to keep his feet on the pedals, or make the handle-bars respond to the balancing of the wheel. In a short time, however, all these movements take place in an effective and graceful manner without any apparent attention being given to them. This efficiency is conditioned by the fact that all these movements have become habitual, or take place largely as reflex acts.

In school also, when the child learns to perform such an act as making the figure 2, the same changes take place. Here an impression must first proceed from the given copy to a sensory centre in the cortex. As yet, however, there is no vital connection established between the sensory centres and the motor centres which must direct the muscles in making the movement. As the movement is attempted, however, faint connections are set up between different centres. With each repetition the connection is made stronger, and the formation of the figure rendered less difficult. So long, however, as the connection is established within the cortex, the movement will not take place except under conscious direction. Ultimately, however, similar connections between sensory and motor neurons may be established in lower centres, whereupon the action will be performed as a reflex act, or without the intervention of a directing act of consciousness. This evidently takes place when a student, in working a problem, can form the figures, while his consciousness is fully occupied with the thought phases of the problem. Thus the neural condition of physical habit is the establishment of easy passages between sensory and motor nerves in centres lower than the cortex.