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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.

Fig. 2—Pyramidal Cell Body. a, Nerve fiber with collaterals.

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).

Fig. 4.—Various Types of Cell Bodies. 1 and 2, Giant pyramidal cell bodies; n, nerve fiber.

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.

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.