composition of nervous tissue. Ordinary nerve-fibers in the living
subject, or when fresh, are cylindrical-shaped filaments of a clear, but
somewhat oily appearance. But soon after death the matter contained in
the fiber coagulates, and then the fiber is seen to consist of an
extremely delicate, structureless, outer membrane, which forms 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