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TWENTY-SEVENTH LECTURE.

Cavities of the Third, Fourth and Fifth Classes.

These include all cavities in the proximate surfaces of
the teeth. Although properly divided into three classes from
the standpoint of differences in technical procedures, the
third including- the ordinary cavities in the incisors and cus-
pids, the fourth those in which an angle of the tooth is in-
volved, and the fifth including the proximate cavities in the
bicuspids and molars, the conditions from the standpoint
of the management of cases may in part be presented to-
gether. The fourth class includes only cavities of the third
in which extension of decay has involved the angle and
calls only for differences in the technical procedures, and
may therefore be eliminated from present consideration.
These are the most destructive decays that occur in the
teeth and occupy more of the dentist's time than any other.
They also call for a greater degree of acuteness and skill
in the management of conditions than any other.
In the order of occurrence these cavities follow or occur
later than those of the first class, but are usually earlier
than those of the second class. It must be understood, how-
ever, that this expresses the general rule, to which there
jare many exceptions. Between the time and the frequency
of the appearance of proximate cavities in the incisors, and
in the bicuspids and molars, there is the utmost variation
in individuals.
In some the incisors will escape entirely or with but one
or two cavities, while the back teeth will have many, and
in others the case will be reversed. In the aggregate the
liability is about equal, though where individual proximate
surfaces are considered the mesial surfaces of the central
incisors are found decayed oftener than any others. In any
of the teeth the mesial surfaces are oftener decayed than
the distal, apparently because they are less rounded in con-
tour. In considering these it must be remembered that
there are two areas of liability—one mesial and one distal
on each tooth—while in the first and second classes there
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