eak 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.
]

263. The Functions