Page:Popular Science Monthly Volume 71.djvu/110

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 It is usually called a Purkinje's cell. It was stained black throughout by what is known as the Golgi silver method, hence shows nothing of its internal structure. After von Kölliker.

spinal cord none of these processes existed, and the amount of the protoplasm in the nerve cell was very much smaller. As development progressed, not only did the protoplasm body grow, but the processes gradually grew out. Some of them branched so as to better receive and collect the impulses; one of them remained single and very much elongated, and acquired a somewhat different structure in order to serve to carry the nervous impulses away. The third picture shows us a section through the spinal cord of an adult fish. It has been treated by a special stain in order to show how certain elements of the spinal cord acquire a modification of their organization by which they are adapted to serve as supports for the nervous elements proper. They play in the microscopic structure the same supporting role which the skeleton performs in the gross anatomy of the body as a whole. They do not take an active part in the nervous functions proper. None of the appearances which this figure offers for our consideration can be recognized in any similar preparation of the embryonic cord. Obviously, then, from the embryonic to the adult state in the spinal cord there occurs a great differentiation. That which was alike in all its parts has been so changed that we can readily see that it consists of many different parts. A striking illustration of this is afforded by the next picture, which represents one of the large nerve cells which occur in the small brain, or cerebellum, that portion of the central nervous system which the physiologists have