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60 THE TECHNICAL PROCEDURES IN FILLING TEETH,

large cavities in the distal surfaces of upper molars the disto-
lingual groove is often approached. In any such case cut to the
groove until it has passed over the ridge to the lingual surface,
when its direction is more to the mesial, and then break away
from it rather suddenly, forming a kind of step, if the enamel of
this portion of the lingual surface is well supported by sound
dentin.
If the mesio-buccal angle of a molar is so decayed as to make
the removal of a considerable part of it necessary, cut to the buc-
cal groove, for if this is not done the intervening portion of
enamel is very likely to break away.
The same rule applies to the triangular grooves of the bicus-
pids that pass to the buccal surface to the mesial and distal of the
buccal cusps. If in any case the angles of these teeth are so
injured that the cavity lines should approach near these grooves,
cut to the grooves.
In any case in which the angle of an incisor must be removed,
cut to the labial groove, whether it be mesial or distal. This is
for the reason that the lines of the grooves are weak lines, and
the enamel is likely to break along these lines.
These cases will sufficiently illustrate this rule.

Excavation of Cavities by Classes.
Note.— The determination of the conditions calling for
filling operations is largely dependent upon pathological pro-
cesses. As it is the intention to confine this book as strictly as
possible to the technical procedures of filling operations, these
pathological processes will not be discussed. Here we can only
take note of the physical conditions presented by individual
teeth, leaving pathological processes related to them to be taken
up at another time.

Class I.— Cavities beginning in structural defects in
the teeth, as pits and fissures.— These occur in the occlusal
surfaces of the molars and bicuspids, in the occlusal half of the
buccal surfaces of the molars, or in the buccal pits, more rarely
in the lingual portion of the disto-lingual groove of the upper
molars, and in the lingual surfaces of the upper incisors ; most
frequent in the laterals.