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ABRASIOlSr OP THE TEETH. 215

In many cases, however, the occlusion of the teeth will be
found to have been permanently impaired by such movement,
and the building of prominent contacts that can not be main-
tained will be useless. Not very infrequently it has been found
that the better treatment will be to widen the interproximal
space enough to prevent lodgment of food. This is always a
grave misfortune, as the power of mastication is permanently
impaired.
Abrasion of the Teeth.
ILLUSTRATIONS: FIGURES 298-310.
By "abrasion" is meant such wear of the surfaces of the
teeth as will occur by the normal processes of mastication. That
this common statement of the case is incorrect, any one may
become satisfied by the careful examination of some hundreds of
individuals, from forty to sixty years old, and tabulating the
condition of the teeth as to abrasion of their occlusal surfaces,
and any other conditions discoverable that may be regarded as
influencing it. After this has been done by a skillful observer,
the verdict will certainly be that there is some unknown influence
responsible for an abnormal amount of abrasion of the teetli of
certain individuals. Yet, it is quite certain that the friction
of mastication controls the form of the abraded surfaces of the
teeth.
It seems unnecessary now to enter into a discussion of that
which promises no good results, and that feature of the subject
will be passed with the statement that there are great differences
among persons as to the amount of wear of the teeth. Among
the great majority of our people there is about a certain degree
of abrasion of the teeth that is apparent in nearly all of the age
of forty years or more. This is not considerable ; and is insuf-
ficient to cause any disfiguration or to require any attention
because of evil results. In a comparativelj^ few individuals there
is very much greater wear ; so much that persons are occasion-
ally seen, the crowns of whose teeth are worn to the level of the
gums at the age of forty. A greater number will be met with,
whose teeth are worn so much as to disfigure the mouth and the
features also, by the closing of the lower jaw too far, interfering
with the position of the lips and shortening the chin. Indeed,
some persons appear almost as those who have lost their teeth,
though there may be a considerable part of the crowns remaining.
In abrasions that progress rapidly, there are certain fea-
tures which show that the use of the teeth in mastication has