e on drinkers, for their wounds do not heal so quickly as in
other people.

When there is too little air, a fire burns slower, and makes a blacker
smoke and more ashes. Alcohol takes some air from the cells of the
body. So they burn with smoke and ashes of the wrong kind. The skin
has to work harder to get rid of these, and sometimes it cannot do it
well. Then the body is poisoned. The alcohol is burned and cannot
poison the body any more. But it causes the body to make poisons, and
so it is to blame. The poisons do great harm to the skin and kidneys.
Alcohol causes more kidney disease than all other things put together.

WHAT WE HAVE LEARNED

1. Little tubes in the skin are always giving off ashes and waste
matters in the perspiration.

2. Perspiration dries on the skin. So the skin must be washed
often.

3. The kidneys get rid of more water and waste matter than the
skin does.

4. Perspiration also gets upon the clothes and bed sheets. These
must be washed too.

5. Dirty water from washing should be thrown out where it cannot
run into a well.

6. The skin is thick and strong and keeps the body from being hurt.

7. The skin is covered with a layer of scales. The scales have no
feeling.

8. The scales form the nails on the ends of the fingers.

9. The scales also form the hair.

CHAPTER XII

THE NERVES, SPINAL CORD, AND BRAIN

=136. Need of nerves.=--The cells of the mouth, stomach, and intestine
digest food; the cells of the liver change the food to blood; the
cells of the heart pump the blood to feed all the cells of the body;
the red blood cells carry air for the cells to breathe; and the cells
of the skin and kidneys carry away the waste of the rest of the cells.
Each set of cells works for all the rest. If the cells of the body
were only tied together, each one would do as it pleased, and no two
would work together. But something tells each cell of the body to work
with the others. The cells all obey the mind. A tiny thread goes to
each cell of the body. Each thread is a _nerve_. The mind and the
cells signal to each other over the nerves. By means of the nerves the
mind makes the cells work together.

[Illustration: =A nerve thread (x400).=

_a_ central conducting fiber.
_b_ covering of fat.]

[Illustration: =A thin slice for the end of a cut nerve (x200).=

_a_ nerve th