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EXCAVATION OF CAVITIKS HY CLASSES. 199

other undesirable substance to enter, the tootli may retain its
color sufficiently for a porcelain restoration. This, however,
requires the utmost care in every detail.

Class 5. Cavities in the Gingival Third — not Pit Cavities
— IN THE Buccal, Labial or Lingual Surfaces
OF THE Teeth".

ILLUSTRATIONS: FIGUUES 281-292.
Of these, the smooth surface cavities of the lingual surfaces
of the teeth are so infreciuent that mention of them seems to be
all that is necessary. The differences in position call for some
differences in instrumentation between these and similar cavi-
ties of the buccal and labial surfaces. While the principles of
their preparation are the same, the lingual surfaces are awk-
wardly situated for the use of instruments and often call for dif-
ferences in the particular instruments employed.
The cavities of the classes treated thus far have had their
beginning in out-of-the-way places, hidden in pits or between
the proximal surfaces of the teeth. Those of the fifth class are
very different. These cavities are all on surfaces that are freely
exposed to view and when clean are without natural covering of
any kind. They begin in the enamel, close to the line of the free
margin of the gum. Hence the term "gingival third cavities."
All are inclined to spread on the surface of the enamel in direc-
tions along the gum margin either way from the starting point.
In cases of the greatest susceptibility to caries, these are inclined
to form a band of decay encircling the tooth along the gum mar-
gin by joining smooth surface decays of the proximal surfaces
at the angles of the teeth. In the ordinary cases seen in prac-
tice, the tendency to spread is in the same directions, but not
quite to the angles of the teeth. This fact forms the real key
to the extensions necessary for the prevention of further spread
of decay in all of the smooth surface cavities ; the proximal as
well as gingival third cavities. With this fact well fixed in the
mind and finally confirmed by careful observation, one should
not go wrong in the extension of cavities of this class for the
prevention of further spreading of caries ou any surface. There-
fore, in the treatment of gingival third cavities, all principal
extensions must be in a direction along the free border of the
gum. In some cases extensions of lesser degree may be required
in other directions, but these will not be considerable.
The gingival third cavities are much inclined to occur in