Page:Popular Science Monthly Volume 43.djvu/396

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brain or spinal cord. Most all the nerve fibers produced within the brain enter the ventral territory, for in this territory we observe not only the fibers which it obviously must include—namely, those which are produced by the nerve cells of the ventral zone—but also the nerve fibers produced by the nerve cells of the dorsal zone. So far as at present known, the nerve cells of the dorsal zone all produce nerve fibers, but these fibers always pass into the ventral division of the nervous system. These fibers of dorsal origin are the chief, perhaps the only ones, which are commissural—that is to say, which pass to the opposite side of the brain; others of these fibers take longitudinal courses within the ventral zone; while still others participate in the formation of the nearest ventral (or anterior) nerve roots. If, therefore, we assume that the sensory nerve impulses are carried into the dorsal zone and there transferred to the medullary nerve cells, we must conclude that from those cells the impulse may be sent along medullary fibers either into the opposite side, or up and down the ventral zone, or into a neighboring nerve root. The center of divergence is the dorsal zone, but the actual divergence of the fibers takes place in the ventral zone.

Although the ventral zone receives medullary fibers and itself produces nerve fibers, it sends, so far as yet observed, no fiber into the dorsal zone, but all the fibers which leave the ventral zone form nerve roots and leave the nervous system altogether. These roots, as we have already learned, are in two sets—the lateral and ventral.

Summary.—The numerous facts which we have marshaled in hasty review so greatly widen our knowledge of the nervous system that it is important to render them as clear as possible. If what has been presented be critically considered, it will be found that what we have gained is an enormous accession of knowledge in regard to the nature, origin, distribution, and connections of nerve fibers. In order to make the typical variations of nerve fibers as evident as possible, I have constructed the accompanying diagram, which is, I think, correct for all which it attempts to give. We notice: First, that the central nervous system is a medullary tube, the walls of which form two dorsal zones and two ventral zones. Second, that every nerve fiber arises from a single cell only, and is nowhere united with any other cell. Third, that every nerve fiber has a branching termination. Fourth, there are three kinds of nerve fibers: (1) Medullary, which arise from the nerve cells of the central nervous system proper; of the medullary fibers three kinds are distinguished—namely, those which pass out to form the ventral root, those which pass-out to form the lateral root, and those which pass as commissures to the opposite side of the tube; there are also medullary fibers which run