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.

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.
]