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22 OPERATIVE DENTISTRY
Second. Smooth surface cavities, which are those occurring on sur-
faces without defects in the enamel, but are habitually unclean.
(See Figs. 3 and 4.)
Cavities are divided according to similarity in line of treatment
into six divisions.
Class One. Those cavities beginning in structural defects. (Pits
and fissures.)
Class Two. Those cavities in the proximal surfaces of bicuspids
and molars.
Class Three. Those cavities in the proximal surfaces of incisors
and cuspids not involving the incisal angle.

Fig. 2.—Defects in enamel.
Class Four. Those cavities in the proximal surfaces of incisors
and cuspids which require the restoration of the incisal angle.
Class Five. Those cavities in the gingival third of the labial,
buccal and lingual surfaces not originating in faults in enamel.
Class Six. Abraded surfaces.
The outside walls of a cavity are those walls placed toAvard the
outside surfaces of a tooth and take the names of the surfaces of
the tooth toward which they are placed, as in an occlusal cavity
the outside walls are buccal, distal, mesial and lingual, while the
fifth or internal wall is the pulpal.
The pulpal wall is that inside wall of a cavity which covers the
pulp and is in a plane at right angles to the long axis of the tooth.