Page:Popular Science Monthly Volume 43.djvu/388

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out from the brain and spinal cord. The brain and spinal cord, since the days of the celebrated investigations of Karl Ernst von Baer, have been identified as modifications of a single long tube, the so-called medullary tube of embryology. This tube, as the embryo advances, gradually increases in complexity, especially in the region of the head, until it is converted into the brain and spinal cord. The complications which occur may be conveniently grouped under four heads—namely, the flexures, the widening of the cavity or its obliteration in a way varying for each region, changes in the thickness of walls, and lastly an extreme differentiation of the microscopic organization. Without detailed explanation it may be readily conceived that by the varying

PSM V43 D388 Brain and spinal cord.jpg

cooperation of these factors great differences arise in the sundry parts of the originally simple medullary tube. On the other hand, in the most fundamental characteristic, the production of nerve fibers, the same principle governs brain and spinal cord alike. There appear very early certain cells, which soon become recognizable as young nerve cells (neuroblasts) because of their size and pointed shape; the pointed end now elongates into a very delicate thread, the nerve fiber, which is at first very short but rapidly lengthens almost like a growing root; the growing fiber takes its course for a certain distance, varying according to circumstances, within the wall of the medullary tube, but ultimately passes outside the tube into the neighboring tissues together with other nerve fibers of similar origin. It must be added that some