ded lengthwise into three nearly
equal parts, which are termed the anterior, lateral, and posterior
columns, by the lines which join together two parallel series of bundles
of nervous filaments, which compose the roots of the spinal nerves. The
roots of those nerves, which are found along that line nearest the
posterior surface of the cord, are termed the posterior roots; those
which spring from the other line are known as the anterior roots.

Several of these anterior and posterior roots, situated at about the
same height on opposite sides of the spinal cord, converge and combine
into what are called the _anterior_ and _posterior bundles_; then two
bundles, anterior and posterior, unite and form the trunk of a spinal
nerve.

The nerve trunks make their way out of the spinal canal through
apertures between the vertebra, called the _inter-vertebral foramina_
and then divide into numerous branches, their ramifications extending
principally to the muscles and the skin. There are thirty-one pairs of
spinal nerves, eight of which are termed cervical, twelve dorsal, five
lumbar, and six sacral, with reference to that part of the cord from
which they originate.

When the cord is divided into transverse sections, it is found that each
half is composed of two kinds of matter, a white substance on the
outside, and a grayish substance in the interior. The _gray matter_, as
it is termed, lies in the form of an irregular crescent, with one end
considerably larger than the other, and having the concave side turned
outwards. The ends of the crescent are termed the _horns_, or _cornua_,
the one pointing forward being called the _anterior cornu_, the other
one the _posterior cornu_. The convex sides of these cornua approach
each other and are united by the bridge, which contains the central
canal.

There is a marked difference in the structure of the gray and the white
matter. The white matter is composed entirely of nerve fibers, held
together by a framework of connective tissue. The gray matter contains a
great number of ganglionic corpuscles, or nerve-cells, in addition to
the nerve-fibers.

When the nerve-trunks are irritated in any manner, whether by pinching,
burning, or the application of electricity, all the muscles which are
supplied with branches from this nerve-trunk immediately contract, and
pain is experienced, the severity of which depends upon the degree of
the irritation; and the pain is attributed to that portion