surface of the
cell-body, to which they are related in function.

The _axon_, or nerve fiber, is a long, slender extension from the
cell-body, which connects with some organ or tissue. It was at one time
described as a distinct nervous element, but later study has shown it to
be an outgrowth from the cell-body. The mon-axonic neurons are so called
from their having but a single axon.

*Di-axonic Neurons.*--Neurons belonging to this class have each a
well-defined cell-body and two axons, but no parts just like the dendrites
of mon-axonic neurons. The cell-body is smooth and rounded, and its axons
extend from it in opposite directions (Fig. 127).

[Fig. 127]

Fig. 127--*Diagram of a di-axonic neuron.* The diagram shows only the
conducting portion of the axon, or axis cylinder.

*Structure of the Axon.*--The axon, or nerve fiber, has practically the
same structure in both classes of neurons, being composed in most cases of
three distinct parts. In the center, and running the entire length of the
axon, is a thread-like body, called the _axis cylinder_ (Fig. 126). The
axis cylinder is present in all axons and is the part essential to their
work. It may be considered as an extension of the protoplasm from the
cell-body. Surrounding the axis cylinder is a thick, whitish-looking
layer, known as the _medullary sheath_, and around this is a thin
covering, called the _primitive sheath_, or neurilemma. The medullary
sheath and the primitive sheath are not, strictly speaking, parts of the
nerve cell, but appear to be growths that have formed around it. Certain
of the axons have no primitive sheath and others are without a medullary
sheath.(97)

*Form and Length of Axons.*--Where the axons terminate they usually
separate into a number of small divisions, thereby increasing the number
of their connections. Certain axons are also observed to give off branches
before the place of termination is reached (Fig. 131). These collateral
branches, by distributing themselves in a manner similar to the main
fiber, greatly extend the influence of a single neuron.

In the matter of length, great variation is found among the axons in
different parts of the body. In certain parts of the brain, for example,
are fibers not more than one one-hundredth of an inch in length, while the
axons that pass all the way from the spinal cord to the toes have a length
of more than three feet. Between these