*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.
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