g both the structure and the working of the
nervous system to keep in mind that it contains _but one fundamental
unit of structure_. This is the neurone. Just as the house is built up
by adding brick upon brick, so brain, cord, nerves and organs of sense
are formed by the union of numberless neurones.

[Illustration: FIG. 6.--Neurones in different stages of development,
from _a_ to _e_. In _a_, the elementary cell body alone is present; in
_c_, a dendrite is shown projecting upward and an axon downward.--After
DONALDSON.]

THE NEURONE.--What, then, is a neurone? What is its structure, its
function, how does it act? A neurone is _a protoplasmic cell, with its
outgrowing fibers_. The cell part of the neurone is of a variety of
shapes, triangular, pyramidal, cylindrical, and irregular. The cells
vary in size from 1/250 to 1/3500 of an inch in diameter. In general the
function of the cell is thought to be to generate the nervous energy
responsible for our consciousness--sensation, memory, reasoning, feeling
and all the rest, and for our movements. The cell also provides for the
nutrition of the fibers.

[Illustration: FIG. 7.--Longitudinal (a) and Transverse (b) section of
nerve fiber. The heavy border represents the medullary, or enveloping
sheath, which becomes thicker in the larger fibers.--After DONALDSON.]

NEURONE FIBERS.--The neurone fibers are of two kinds, _dendrites_ and
_axons_. The dendrites are comparatively large in diameter, branch
freely, like the branches of a tree, and extend but a relatively short
distance from the parent cell. Axons are slender, and branch but little,
and then approximately at right angles. They reach a much greater
distance from the cell body than the dendrites. Neurones vary greatly in
length. Some of those found in the spinal cord and brain are not more
than 1/12 of an inch long, while others which reach from the extremities
to the cord, measure several feet. Both dendrites and axons are of
diameter so small as to be invisible except under the microscope.

NEUROGLIA.--Out of this simple structural element, the neurone, the
entire nervous system is built. True, the neurones are held in place,
and perhaps insulated, by a kind of soft cement called _neuroglia_. But
this seems to possess no strictly nervous function. The number of the
microscopic neurones required to make up the mass of the brain, cord and
peripheral nervous system is far beyond our mental grasp. It is computed
that