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COMPARATIVE LIABILITY TO HECAY. 63

cases three, or four kinds of metals are employed in
filling teeth of the same mouth ; in some, fillings of
one metal and a plate of another ; and in others, plates
of so low a carat are used that they oxydize rapidly
in the mouth, without the aid of any other metal.

COMPARATIVE LIABILITY TO DECAY.

All classes of teeth are not alike liable to decay.
Their difference in this respect may arise from a dis-
similarity in their organic structure, the best organized
being the most capable of resisting disease ; or from a
concentration of the destructive agency upon the
tooth first affected. The first molars are much more
liable to decay than any other teeth, since they are

less perfectly developed than those formed at a later
period of life. They are the first permanent teeth
erupted, and are subjected to all the irritating condi-
tions consequent on the removal of the temporary, and
the development and eruption of the permanent teeth.
But these conditions, in many cases, produce no ap-
parent injury upon them, they maintaining their integ-
rity till all the other permanent teeth appear, and then
decaying earlier than any others. In such cases,
the decay is a result of influences more potent than

those occurring on the eruption of the other teeth.
After the first, the second molars are most subject