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.