many branches
or extensions from the main body of the cell.

[Illustration: FIG. 4. Body of a nerve-cell of the spinal cord,
specially stained so as to show the minute structure. (Schaefer's
_Histology_.)]

[Illustration: FIG. 5. A large nerve-cell from the spinal cord of the
ox, magnified 175 diameters. (Schaefer.)]

[Illustration: FIG. 6. A cell of another form, from the superficial or
outer part of the greater brain (cortex cerebri). The great amount of
branching is suggestive of the power to receive and to transmit
nervous influences (impulses) from various other cells; in other
words, complexity of structure suggests a corresponding complexity of
function.]

[Illustration: FIG. 7. Representation of the manner in which a nerve
is seen to terminate in a muscle, such ending being one form of
"nerve-ending" termed a "muscle plate." It tends to emphasize the
close relationship existing between muscle and nerve, and to justify
the expression "neuro-muscular mechanism," the nervous system being as
important for movements as the muscles. (Schaefer's _Histology_.)]

[Illustration: FIG. 8. Three muscle-fibres lying beside each other,
with the small blood-vessels (capillaries) around and between them.
Such are the appearances presented under the microscope by skeletal or
striped muscles such as those of the larynx. (Schaefer.)]

It may be said, in general terms, that the nervous _centres_, the
brain and the spinal cord, which are parts of one anatomical whole,
are characterized by the presence of the cell-bodies as well as their
extensions, while nerves consist only of the extensions or arms of the
cell-bodies. The nerve-cell whose body is in the top of the brain may
have an extension or arm which may reach practically to the end of the
spinal cord, and there make communication with another cell whose arm,
in turn, may reach as far as the toe. Such nerve arms or extensions
constitute the _nerve-fibres_, and bundles of these _nerves_, or
_nerve-trunks_.

Usually nerve-fibres make connection with the cells of an organ by a
special modification of structure known as a _nerve-ending_. A nervous
message or influence (_nerve-impulse_) may pass either to the
centre--_i.e._, toward a cell-body--or from it; in other words, a
nervous impulse may originate in the centre or in some organ more or
less distant from it; a nervous impulse may be _central_ or
_peripheral_. Nearly all central impulses, we now know, arise because
of