read.
_b_ connective tissue binding the threads into a cord.]

=137. Nerve messages.=--The nerve threads run in bundles and form
nerves large enough to be seen. The mind uses the nerves to tell the
cells to do work. It tells the muscles to move the arms and legs. It
tells the heart to beat and stomach to pour out gastric juice; and it
tells each of the cells to eat.

The cells also send word over the nerves to the mind. They tell the
mind when they are touching anything, and whether it is hard, or
smooth, or hot, and many other things about it. The cells also tell
the mind if they need more food, or are tired.

The nerves are always carrying messages to and from the cells. The
cells depend upon these messages to tell them when and how to work. If
the nerve of any part of the body is hurt or cut, we cannot feel with
the part or move it, and its cells do not act in the right way. We do
not feel the nerves while they are carrying the messages. We wish the
cells of the arm to work, and they work, but we do not feel the
message as it goes from the mind to the cells of the arm.

[Illustration: =A thin slice from the spinal cord with the cells and
nerves magnified 200 diameters.=

_a_ cells in the gray matter.
_b_ fibers in the gray matter.
_c_ nerve threads in the white matter.]

=138. The spinal cord.=--The nerves start inside the backbone. The
backbone is hollow. It has a soft, white cord inside, as thick as the
little finger. Part of the mind lives in this cord. The cord is called
the _spinal cord_. Some of the nerves start from cells of the spinal
cord. These cells send word to the muscles to move and to all the
cells of the body to eat and grow. They also send word to the arteries
to carry the right amount of blood to the cells.

From the nerves the spinal cord gets word when something hurts any
part of the body. You may put your finger on a sharp pin. The spinal
cord feels the prick, and quickly sends word to snatch the finger
away. So the finger is taken away before you really feel the prick.
When some one sticks a pin into you, you cannot help jumping. This is
because the spinal cord sends word for you to jump away from the pin
before it can harm you much. Thus the spinal cord keeps the body from
being hurt. It acts while we are asleep as well as when we are awake.

=139. Need of a spinal cord.=--We do not feel the spinal cord acting,
and we cannot keep it from acting. It tells the cells when to eat