Читать книгу Psychology: an elementary text-book - Hermann Ebbinghaus - Страница 10

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´.

Fig. 7.—Diagram of Nervous Architecture: Reflex Arches connected by a Low Nerve Center. (From Psychological Review, 15, 1908.)

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.

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.