s a tube
through the center of which runs the _axis-cylinder_. Interposed between
the axis-cylinder and this tube, there is a fluid, containing a
considerable quantity of fatty matter, from which is deposited a highly
refracting substance which lines the tube. There are two sets of
nerve-fibers, those which transmit sensory impulses, called _afferent_
or _sensory_ nerves, and those which transmit motor impulses, called
_efferent_ or _motor_ nerves. The fibers when collected in bundles are
termed nerve trunks. All the larger nerve-fibers lie side by side in the
nerve-trunks, and are bound together by delicate connective tissue,
enclosed in a sheath of the same material, termed the _neurilemma_. The
nerve-fibers in the trunks of the nerves remain perfectly distinct and
disconnected from one another, and seldom, or never, divide throughout
their entire length. However, where the nerves enter the nerve-centers,
and near their outer terminations, the nerve-fibres often divide into
branches, or at least gradually diminish in size, until, finally, the
axis-cylinder, and the sheath with its fluid contents, are no longer
distinguishable. The investing membrane is continuous from the origin to
the termination of the nerve-trunk.

[Illustration: Fig. 55.
Division of a
nerve, showing a
portion of a nervous
trunk (_a_)
and separation of
its filaments (_b, c, d, e_.)]

In the brain and spinal cord the nerve-fibers often terminate in minute
masses of a gray or ash-colored granular substance, termed _ganglia_, or
_ganglionic corpuscles_.

The ganglia are cellular corpuscles of irregular form, and possess
fibrous appendages, which serve to connect them with one another. These
ganglia form the cortical covering of the brain, and are also found in
the interior of the spinal cord. According to Koelliker, the larger of
these nerve-cells measure only 1/200 of an inch in diameter. The brain
is chiefly composed of nervous ganglia.

Nerves are classified with reference to their origin, as
_cerebral_--those originating in the brain, and _spinal_--those
originating in the spinal cord.

There are two sets of nerves and nerve-centers, which are intimately
connected, but which can be more conveniently studied apart. These are
the _cerebro-spinal_ system, consisting of the cerebro-spinal axis, and
the cerebral and spinal nerves; and the _sympathetic_ system, consisting
of the chain of sympathetic ganglia, the nerves which they give off, and