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EXCAVATION OF CAVITIES BY CLASSES. 145

What extension for prevention does not mean. Exten-
sLou for prevention does not mean deep cavities in any case.
The whole intention is to prevent the beginning of decay in the
snrface of tlie enamel. It does not have any reference to caries
of dentin whatever. The shallowest cavity that will give suf-
ficient stability to a filling answers the requirement. A much
shallower proximal cavity answers the purpose when the occlusal
step is used. Under all circumstances in filling teeth, the object
should be to make cavities only deep enough to give stability
to the filling, except as deeper cavities are required by removal
of the last trace of decayed material.
Cutting away frail walls to points or lines of sufficient
strength, should in no case be confounded with extension for
prevention. This kind of cutting unfortunately leads to the
laying of cavity margins on lines less safe by the necessity of
placing the line at which the cavity margin approaches or crosses
the free margin of the gum on a buccal or labial surface. This
is always a dangerous locality as compared with the angle of
the tooth.
Further illustrations of the use of the principle of exten-
sion for prevention will be found in nearly every case presented.
Early discovery of cavities. The discovery of proximal
cavities while they are yet very small is more difficult than any
other. They are hidden away between the proximal surfaces
of the teeth and can not be seen. They must be discovered at the
time when the enamel rods have just begun to fall away so that
the exploring tine will enter, if they are treated at the time at
which the least injury will be done to the teeth. Observation
of the general practice of dentistry leads to the conclusion that
comparatively few of these cavities are discovered and treated
so early, and yet there is no doubt whatever but that this early
treatment may be done with much less actual injury to the teeth.
The examination is often difficult and requires considerable edu-
cation of the sense of touch in the use of the exploring tine.
Many persons have much difficulty in distinguishing between
the catching of the point of the explorer between the proximal
surfaces of the teeth near the contact point and the entry of the
point into a cavity of slight depth. Not infrequently it will be
necessary to put on the rubber dam and the separator and lift
the teeth slightly apart to be assured of the actual condition.
After the decay has extended along the dento-enamel junction,
undermining the marginal ridge until the enamel begins to break,
so that the patient discovers the cavity, it will often be found

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