times each day
as it is to wash the hands and face. A few germs of diphtheria, sore
throat, or tuberculosis are likely to get into the mouth any day, but
if the mouth and teeth are well washed with a brush morning and night,
the germs will not have time to grow and cause sickness.

=The Teeth.=--The first twenty teeth that appear are called the _milk
set_. The eight front teeth grow out during the first year of life and
back of these twelve others appear during the second year. Between the
seventh and the tenth year all of the milk teeth are lost because
others grow beneath them and push them out.

[Illustration: FIG. 31.--The full set of teeth on the right side at
twelve years of age. The numbers show at what year of age each one
grows out of the gum.]

The first four teeth of the second set appear in the sixth year, just
behind the last milk teeth (Fig. 30). These teeth should be watched
very closely and at the first sign of decay you should go to the
dentist. As the milk teeth get loose and come out, the second set of
teeth take their places.

If you are ten or eleven years old, you should have twelve good teeth
in the upper jaw and the same number below. The last ones to break
through the gums are the four wisdom teeth at the back of the mouth.
They appear after the seventeenth year.

The front teeth are called _incisors_ because they are used to cut the
food. The back teeth are named _molars_ because they are used in
grinding the food.

[Illustration: FIG. 32.--Photograph of teeth not kept clean, showing
how germs and a sour substance called acid eat holes in them and thus
cause decay and toothache.]

=Toothache.=--Toothache is a common ailment, and yet it can be
entirely prevented. A tooth does not ache until it has a hole in it.
The tender nerve within gives us warning that it is being hurt. The
dentist can stop the ache and mend the tooth so that it will not ache
again. Look at your teeth every month and feel about them with a
wooden tooth-pick to know when the decay begins. If the little holes
are mended as soon as found, you will never have toothache, and you
can keep your teeth as long as you live.

[Illustration: FIG. 33.--Slice down through a tooth showing _f_, the
enamel, and _d_, the soft pulp with nerves and blood tubes from the
root at _h_.]

=How to keep the Teeth Sound.=--Every tooth is covered with a layer of
hard shining substance called _enamel_ (Fig. 33). So long as this is
unbroken t