it from injury.

The surface of the brain is very uneven. There are a great many folds
separated by grooves. Some of these are more than an inch deep.

[Illustration: FIG. 86.--The under side of the brain and the spinal cord
with the chief nerves of one side of the body viewed from in front.]

=Parts of the Brain.=--The brain is divided into three chief parts.
The upper and larger part is called the _big brain_ or _cerebrum_.
The lower part behind is the _little brain_ or _cerebellum_. The part
under the little brain and round like the thumb is the _stem_ of the
brain. It connects the larger parts of the brain with the spinal cord.

The big brain is partly separated into halves by a deep cut called a
_fissure_. Each half is a _hemisphere_.

The outer layer of the brain is gray. It is made of millions of tiny
lumps of matter which are the bodies of nerve cells. These are
connected by threads much finer than hairs with other parts of the
brain and spinal cord. Over these threads called _nerve fibers_ one
cell can talk to another somewhat as we talk over a telephone wire.

[Illustration: FIG. 87.--Side of the skull cut away to show the brain.
_B_, backbone.]

=The Spinal Cord.=--This is a bundle of nerve matter about as thick as
your finger. It extends from the stem of the brain down the canal in
the backbone. The outer layer of the spinal cord is white because it
is made of the tiny threads, _nerve fibers_. The inner part is made of
the bodies of nerve cells and therefore looks gray. The fibers are
branching threads from the cells in the cord and brain.

=The Message Carriers or Nerve Fibers.=--In order that the managers may
send messages, these fine threads, the nerve fibers, extend from them to
all parts of the body. In many places from five to five hundred or more
of these fibers are united in one white cord called a _nerve_.

Twelve pairs of nerves are joined to the under side of the brain and
thirty-one pairs are connected with the spinal cord (Fig. 86). The
nerves of the brain branch to all parts of the head and neck, and one
pair goes down to the lungs, heart, and stomach. The nerves connected
with the spinal cord branch to every part of the muscles, bones, and
skin of the arms, trunk, and legs.

=How the Nerves do their Work.=--On a telephone wire we can send a
message in either direction. A message can travel on a nerve in only
one direction. For this reason there must be two kinds of nerves. One
kind