Page:Popular Science Monthly Volume 71.djvu/109

Rh the adult. I will have thrown upon the screen for you a succession of pictures illustrating various adult structures. The first is, however, a section of the embryonic spinal cord in which you can see that much of the simple character of the embryonic cells is still kept. All parts of the spinal cord, as the picture shows, are very much alike, and the nuclei of the cells composing the spinal cord at this stage are all essentially similar in appearance. What a contrast this forms with our next picture, which shows us an isolated so-called motor nerve cell from the adult spinal cord. It owes its name motor to the fact that it produces a nerve fiber by which motor impulses are conveyed from the spinal cord to the muscles of the body. The cell has numerous elongated branching processes stretching out in various directions, but all leading back towards the central body in which the nucleus situated. These are the processes which serve to carry in the nervous impulses from the periphery towards the center of the cell, impulses which in large part, if not exclusively, are gathered up from other nerve cells which act on the motor element. At one point there runs out a single process of a different character. It is the true nerve fiber, and forms the axis, as it was formerly termed; or axon, as it is at present more usually named, of the nerve fiber as we encounter it in an ordinary nerve. This single threadlike prolongation of the nerve cell is likewise constituted by the living protoplasm and serves to carry the impulses away from the cell body and transmit them ultimately to the muscle fibers which are to be stimulated to contraction. In the embryonic