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 of the body
to which the filaments of the nerve-trunk are distributed. Thus, persons
who have lost limbs often complain in cold weather of an uneasiness or
pain, which they locate in the fingers or toes of the limb which has
been amputated, and which is caused by the cold producing an irritation
of the nerve-trunk, the filaments, or fibers of which, supplied the
fingers or toes of the lost member.

On the other hand, if the anterior bundle of nerve-fibers given off from