On the Generation of Animals 789a7
"They are shed, rather they have been formed, partly because it is better so
(for what is sharp is soon blunted, so that fresh relay is needed for the
work, whereas the flat teeth cannot be blunted but are only smoothed in time by
wearing down), partly from necessity because, while the roots of the grinders
are fixed where the jaw is flat and the bone strong, those of the front teeth
are in a thin part, so that they are week and easily moved. They grow again
because they are shed while the bone is still growing and the animal is still
young enough to grow teeth. A proof this is that even the flat teeth grow for
a long time, the last of them cutting the gum at about twenty years of age;
indeed in some cases the last teeth have been grown in quite old age. This is
because here is much nutriment in the broad part of the bones, whereas the
front part being thin soon reaches perfection and no residual matter is found
in it, the nutriment being consumed in its own growth."