of them which is in the jaw is called the _root_; and the _incisors_,
which cannot work hard because, like the gig-horses, they have but
little resisting power, possess only small and short roots; whereas the
_canines,_ whose duty it is to tear the food sideways, would run the
risk of being dragged out and left sticking in the substances they are
at work upon, if they were not well secured; these, therefore, have
roots which go much deeper into the jaw, and in consequence of this they
give us more pain than the others when the dentist extracts them: those
famous _eye-teeth_, which so terrify people on such occasions, are the
_canines_ of the upper jaw, and lie, in fact, just below the eye.

The _molars_ meanwhile would be in danger of being shaken in the
sideway movement, while chewing: so they do as you would do if you
were pushed aside. Now you would throw out your feet right and left
in order to steady yourself, and thus the molars, which have always
two roots, throw them out right and left for the same purpose. Some
have three, some four, and they require no less for the business they
have to do.

Above the root comes what is called the crown; that is the part of the
tooth which is exposed to the air; the part which does the work, and
which bears the brunt of all the rubbing. Now, however hard it may be,
it would soon end in being worn out by all this fun if it were not
covered by a still harder substance, which is called _enamel_.
The _enamel_ which forms the coating of china plates, and which
you can easily distinguish by examining a broken plate, will give you
a very exact idea of it. It is this enamel which gives the teeth the
polish and brilliancy we so much admire, and it is desirable to be
very careful of it, not out of vanity, though there is no objection
to a little vanity on the subject, but because the enamel is