Page:Popular Science Monthly Volume 43.djvu/389

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375
STRUCTURAL PLAN OF THE HUMAN BRAIN.

of the nerve fibers are of the Golgi type—that is to say, they end as well as begin within the central nervous system. The bundle of nerve fibers which pass out together constitute a nerve, or, to speak more correctly, a nerve root. So far as yet observed no exception occurs; therefore we may safely assert that every nerve cell of the brain or spinal cord produces one nerve fiber and only one, and this fiber grows out from the nervous system into the tissues of the body. The fiber is single at its origin, but since we always find the peripheral fibers branching, we may add that the fiber is multiple at its termination. The nerve cells acquire also other secondary branches—the so-called protoplasmatic processes or dendrites—which grow out from the cells, but are not nerve fibers and are confined in their growth to the nervous tissue itself. The secondary branches present highly characteristic variations in the different regions of the brain, as described in the text-books.

By the ganglionic portion we now understand the nerve cells which lie in little groups outside of the medullary tube. These cells produce fibers, which grow in two directions—on the one side into the brain or spinal cord, on the other away from the brain and cord into other tissues and organs. It has been observed that the ganglionic nerve cells elongate and become spindle-shaped; each pointed end of the cell grows out into a nerve fiber; as the nerve cell connects the two fibers, we may describe the actual condition accurately as resulting in a single nerve fiber, which has a nerve cell interpolated in its course. Each group of nerve cells forms a bundle of nerve fibers, which constitute the posterior (or so-called dorsal or sensory) root of the anatomists. If we follow a ganglionic fiber into the spinal cord or brain, we find that it forms two branches, as first recorded by Ramon y Cajal, a distinguished Spanish histologist; of these two branches, one runs upward, or in the brain forward, and the other runs downward, or in the brain backward; each fork gives off secondary branches (collaterals), that ramify still further, and are all situated within the central nervous system proper. If we study the termination of the ganglionic fiber at its other end—that is to say, in the tissues or organs—we find that there also there occur several ramifications. These fibers, like the medullary fibers, have each a single origin, but, unlike the medullary fibers, have two sets of multiple terminations. Although both the peripheral and central terminations have been carefully studied, they have never been found connected with other structures or cells, but only to be in contact with them.

The true history of the ganglia and their nerve fibers has been elucidated chiefly through the masterly researches of Wilhelm His, Professor of Anatomy at Leipsic, who is the recognized highest living authority on the development of man. This addition