Psychology: an elementary text-book

by Hermann Ebbinghaus

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´.