Some nerve fibers have no inner sheath (medullary), the outer alone protecting the axis cylinder. These are known as the non-medullary fibers. They are gray, while the ordinary medullary fibers are white in appearance. The white nerve fibers form the white part of the brain and of the spinal cord, and the greater part of the cerebro-spinal nerves. The gray fibers occur chiefly in branches from the sympathetic ganglia, though found to some extent in the nerves of the cerebro-spinal system. In a general way, the nerve fibers resemble an electric cable wire with its central rod of copper, and its outer non-conducting layer of silk or gutta percha. Like the copper rod, the axis cylinder along which the nerve impulse travels is the essential part of a nerve fiber. In a cut nerve this cylinder projects like the wick of a candle. It is really the continuation of a process of a nerve cell. Thus the nerve cells and nerve fibers are related, in that the process of one is the axis cylinder and essential part of the other. The separate microscopic threads or fibers, bound together in cords of variable size, form the nerves. Each strand or cord is surrounded and protected by its own sheath of connective tissue, made up of nerves. According to its size a nerve may have one or many of these strands. The whole nerve, not unlike a minute tendon in appearance, is covered by a dense sheath of fibrous tissue, in which the blood-vessels and lymphatics are distributed to the nerve fibers. Illustration: Fig. 112.—Medullated Nerve Fibers. A, a medullated nerve fiber, showing the subdivision of the medullary sheath into cylindrical sections imbricated with their ends, a nerve corpuscle with an oval nucleus is seen between the neurilemma and the medullary sheath; B, a medullated nerve fiber at a node or constriction of Ranvier, the axis cylinder passes uninterruptedly from one segment into the other, but the medullary sheath is interrupted. 263. The Functions of the Nerve Cells and Nerve Fibers. The nerve cells are a highly active mass of living material. They find their nourishment in the blood, which is supplied to them in abundance. The blood not only serves as nourishment, but also supplies new material, as it were, for the cells to work over for their own force or energy. Thus we may think of the nerve cells as a sort of a miniature manufactory, deriving their material from the blood, and developing from it nervous energy. The nerve fibers, on the other hand, are conductors of nervous energy. They furnish a pathway along which the nerve energy generated by the cells may travel. Made up as they are of living nerve substance, the fibers can also generate energy, yet it is their special function to conduct influences to and from the cells.