261. Nerve Cells. However complicated the structure of nerve tissue in man seems to be, it is found to consist of only two different elements, nerve cells and nerve fibers. These are associated and combined in many ways. They are arranged in distinct masses called nerve centers, or in the form of cords known as nerves. The former are made up of nerve fibers; the latter of both cells and fibers. Illustration: Fig. 110.—Nerve Cells from the Spinal Cord. Nerve cells, which may be regarded as the central organs of the nerve fibers, consist of masses of cell protoplasm, with a large _nucleus_ and _nucleolus_. They bear a general resemblance to other cells, but vary much in size and shape. Nerve cells grow, become active, and die, as do other cells. A number of processes branch off from them, some cells giving one or two, others many. The various kinds of nerve cells differ much in the shape and number of processes. One of the processes is a strand which becomes continuous with the axis cylinder of the nerve fibers; that is, the axis cylinders of all nerve fibers are joined in one place or another with at least one cell. Each part of this system has its own characteristic cell. Thus we have in the spinal cord the large, irregular cells with many processes, and in the brain proper the three-sided cells with a process jutting out from each corner. So characteristic are these forms of cells, that any particular part of nerve structure may be identified by the kind of cells seen under the microscope. Nerve cells and nerve fibers are often arranged in groups, the various cells of the groups communicating with one another. This clustered arrangement is called a nerve center. 262. Nerve Fibers. The nerve fibers, the essential elements of the nerves, somewhat resemble tubes filled with a clear, jelly-like substance. They consist of a rod, or central core, continuous throughout the whole length of the nerve, called the axis cylinder. This core is surrounded by the white substance of Schwann, or medullary sheath, which gives the nerve its characteristic ivory-white appearance. The whole is enclosed in a thin, delicate sheath, known as neurilemma. Illustration: Fig. 111.—Nerve Cells from the Gray Matter of the Brain. The axis cylinder generally passes without any break from the nerve centers to the end of the fibers.[40] The outer sheath (neurilemma) is also continuous throughout the length of the fibers. The medullary sheath, on the other hand, is broken at intervals of about 1/25 of an inch, and at the same intervals nuclei are found along the fiber, around each of which is a minute protoplasmic mass. Between each pair of nuclei the sheath is interrupted. This point is known as the _node of Ranvier_.