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208 PATHOLOGY OF THE HAED TISSUES OF THE TEETH.
change of local conditions. Then these become dark in color and
so remain without further change. These should not be inter-
fered with, as they are just as safe without any filling whatever.
Cavities of the Second, Third and Fourth Classes."
These include all cavities in the proximal surfaces of the
teeth. Although properly divided into three classes from the
standpoint of differences in technical procedures, the second
including the proximal cavities in the bicuspids and molars, the
third including the ordinary cavities in the incisors and cuspids,
and the fourth those in which an angle of the tooth is involved,
the conditions from the standpoint of the management of cases
may in part be presented together. The fourth class includes
only cavities of the third, in which extension of decay has
involved the angle of an incisor and calls only for differences in
the technical procedures. It may therefore be eliminated from
present consideration. Decays of the proximal surfaces 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 con-
ditions than any other.
In the order of occurrence, these cavities follow or occur
later than those in pits and fissures, but are usually earlier than
those occurring in buccal and labial surfaces. It must be under-
stood, however, that this expresses the general rule found by
making averages from many recorded cases and to which there
are many exceptions. Between the time and the frequency of
the appearance of proximal cavities in the incisors and in the
bicuspids and molars, there is the utmost variation in different
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 proximal sur-
faces are considered, the mesial surfaces of the central incisors
are found decayed oftener than any others. The broad, flat
mesial surfaces of the first molars stand next in the frequency
of decay. In any of the teeth, except the first bicuspid, the mesial
surfaces are oftener decayed than the distal surfaces, appar-
ently because they are less rounded in contour. 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 fifth classes there is but one, except as buccal pits in some of
the molars furnish a second. Therefore, if the liability were