t the white substance itself, in each half of the
cord, is divided by the horns of the gray matter and by fibers passing
from them into three parts, which are known as the anterior,
posterior, and lateral columns.

Experiment 130. Procure at the market an uninjured piece of the
spinal cord from the loin of mutton or the sirloin or the rib of beef.
After noting its general character while fresh, put it to soak in dilute
alcohol, until it is sufficiently hard to be cut in sections.

274. The Spinal Nerves. From the gray matter on each side of the
spinal cord 31 spinal nerves are given off and distributed chiefly to
the muscles and the skin. They pass out at regular intervals on each side
of the canal, by small openings between the vertebrae. Having escaped from
the spine, they pass backwards and forwards, ramifying in the soft parts
of the body. The first pair pass out between the skull and the atlas, the
next between the atlas and the axis, and so on down the canal. The eighth
pair, called _cervical_, pass out in the region of the neck; twelve,
called _dorsal_, in the region of the ribs; five are _lumbar_, and five
_sacral_, while the last pair leave the cord near the coccyx.

Each spinal nerve has two roots, one from the anterior, the other
from the posterior portion of the cord. These unite and run side by
side, forming as they pass between the vertebrae one silvery thread, or
nerve trunk. Although bound up in one bundle, the nerve fibers of the two
roots remain quite distinct, and perform two entirely different functions.

After leaving the spinal cord, each nerve divides again and again into
finer and finer threads. These minute branches are distributed through the
muscles, and terminate on the surface of the body. The anterior roots
become motor nerves, their branches being distributed to certain
muscles of the body, to control their movements. The posterior roots
develop into sensory nerves, their branches being distributed through
the skin and over the surface of the body to become nerves of touch. In
brief, the spinal nerves divide and subdivide, to reach with their twigs
all parts of the body, and provide every tissue with a nerve center, a
station from which messages may be sent to the brain.

[Illustration: Fig. 118.--Side View of the Spinal Cord. (Showing the
fissures and columns.)

A, anterior median fissure;
B, posterior median fissure;
C, anterior lateral fissure;
D, posterior latera