like branches, while its axon is often several inches
or even feet in length. The axon is the "slender thread", just spoken
of as analogous to the single telephone wire. A nerve is composed of
axons. [Footnote: The axon is always protected or insulated by a
sheath, and axon and sheath, taken together, are often called a "nerve
fiber".] The "white matter" of the brain and cord is composed of
axons. Axons afford the means of communication between the nerve
centers and the muscles and sense organs, and between one nerve center
and another.

The axons which make up the motor nerves are branches of nerve cells
situated in the cord and brain stem; they extend from the reflex
center for any muscle out to and into that muscle and make very close
connection with the muscle substance. A nerve current, starting from
the nerve cells in the reflex center, runs rapidly along the axons to
the muscle and arouses it to activity.

The axons which make up the optic nerve, or nerve of sight, are
branches of nerve cells in the eye, and extend into the brain stem.
Light striking the eye starts nerve currents, which run along these
axons into the brain stem. Similarly, the axons of the nerve of smell
are branches of cells in the nose.

The remainder of the sensory axons are branches of nerve cells that
lie in little bunches close alongside the cord or {33} brain stem.
These cells have no dendrites, but their axon, dividing, reaches in
one direction out to a sense organ and in the other direction into the
cord or brain stem, and thus connects the sense organ with its "lower
center".

[Illustration: Fig. 5.--Sensory and motor axons, and their nerve
cells. The arrows indicate the direction of conduction. (Figure text:
eye, brain stem, skin, cord, muscle)]

Where an axon terminates, it broadens out into a thin plate, or breaks
up into a tuft of very fine branches ( the "end-brush"), and by this
means makes close contact with the muscle, the sense organ, or the
neurone with which it connects.

{34}

The Synapse

Now let us consider the mode of connection between one neurone and
another in a nerve center. The axon of one neurone, through its
end-brush, is in close contact with the dendrites of another neurone.
There is contact, but no actual growing-together; the two neurones
remain distinct, and this contact or junction of two neurones is
called a "synapse". The synapse, then, is not a thing, but simply a
junction between two neuro