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.

[Illustration: Fig. 113.--Non-Medullated Fibers.

Two nerve fibers, showing the nodes or constrictions of Ranvier and the
axis cylinder. The medullary sheath has been dissolved away. The deeply
stained oblong nuclei indicate the nerve corpuscles within the
neurilemma.]

264. The Nervous System Compared to a Telegraphic System. In men and
other highly organized animals, nerves are found in nearly every tissue
and organ of the body. They penetrate the most minute muscular fibers;
they are closely connected with the cells of the glands, and are found in
the coats of even the smallest blood-vessels. They are among the chief
factors of the structure of the sense organs, and ramify through the skin.
Thus the nervous system is the system of organs through the functions of
which we are brought into relation with the world around us. When we hear,
our ears are bringing us into relation with the outer world. So sight
opens up to us another gateway of knowledge.

It will help us the better to understand the complicated functions of the
nervous system, if we compare it to a telegraph line. The brain is the
main office, and the multitudes of nerve fibers branching off to all parts
of the body are the wires. By means of these, nerve messages are
constantly being sent to the brain to inform it of what is going on in
various parts of the body, and asking what is to be done in each case. The
brain, on receiving the intelligence, at once sends back the required
instructions. Countless messages are sent to and fro with unerring
accuracy and marvelous rapidity.

Thus, when we accidentally pick up something hot, it is instantly
dropped. A nerve impu