y in the
stomach. Then their stomachs ache, and they are sick. Eating too fast
and too much makes children sick oftener than anything else.

Birds swallow their food whole, for they have no teeth. Instead, a
strong gizzard inside grinds the food. We have no gizzards, and so we
must grind our food with our teeth.

=15. Teeth.=--We have two kinds of teeth. The front teeth are sharp
and cut the food; the back teeth are flat and rough and grind it. If
you bite nuts or other hard things you may break off a little piece of
a tooth. Then the tooth may decay and ache.

After you eat, some food will sometimes stick to the teeth. Then it
may decay and make your breath smell bad. After each meal always pick
the teeth with a wooden toothpick. Your teeth will also get dirty and
become stained unless you clean them. Always brush your teeth with
water every morning. This will also keep them from decaying.

[Illustration: =Digestive organs of a bird.=

_a_ esophagus or swallowing tube.
_b_ crop or bag for carrying food.
_c_ stomach.
_d_ intestine.
_e_ gizzard or food grinder.]

=16. Swallowing.=--When food has been chewed and mixed with saliva
until it is a paste, it is ready to be swallowed. The tongue pushes
the food into a bag just back of the mouth. We call the bag the
_pharynx_. Then the pharynx squeezes it down a long tube and into the
stomach. The nose and windpipe also open into this bag, but both are
closed by little doors while we swallow. We cannot breathe while we
swallow. If the doors are not shut tightly, some food gets into the
windpipe and chokes us.

WHAT WE HAVE LEARNED

1. We eat to feed the cells of the body.

2. All food must be made into blood.

3. Changing food to blood is digestion.

4. Cooking softens food and makes it taste good.

5. Food is ground fine in the mouth, and mixed with saliva to
form a paste. Some of its starch is changed to sugar.

6. If food is only half chewed the stomach has to grind it.

7. When we swallow, the tongue pushes the food into a bag back of
the mouth and the bag squeezes it down a long tube to the
stomach.

CHAPTER IV

DIGESTION IN THE STOMACH

=17. The stomach.=--When food is swallowed it goes to the stomach. The
stomach is a thin bag. In a man it holds about three pints. Like the
mouth, it does three things to the food.

[Illustration: =Gastric glands in the stomach (x2