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74 OPERATIVE DENTISTRY
that the cavity margins may be in full view as they pass under the
gum.
The Incisal Outline should be carried incisally until the margin
of the filling will be permanently in view, with a space sufficient
to admit of the free use of the tooth brush on the margin. This
Avould, in many instances, carry the margin beyond the incisal
edge and make a Class Four cavity and is only avoided by separa-
tion and filling of the cavity to a slightly excess contour.
The Labial Outline should be carried into the labial embrasure
until the margins are in full view. The enamel should be split
away until full length rods are obtained. On account of the ex-
posed location of these cavities the esthetic reasons demand as lit-
tle cutting labially as possible. As this margin is practically re-
moved from the stress of occlusion it is not essential that the enamel
be supported by dentine in every instance. However, care should
be taken that the rods are full length and that all rods are re-
moved where there has been a backward decay as shown by a
whitened powder-like condition at their dentinal ends.
Additional Extension for esthetic reasons is sometimes required
ill the labial embrasure. This is more often true in the mesial cavi-
ties Avherein the teeth are angular in form and present surfaces
that are quite flat, resulting in a very square or prominent mesio-
labial angle. In such cases the outline should be carried over the
angle and into the labial surface, that the metal may be brought
into the light, otherwise the completed filling will have the appear-
ance of a decay or dark spot on the tooth.
The Lingual Outline must be carried into the lingual embrasure
sufficiently to be brought into full view in all cases.
In the case of teeth of rounded form this will not alwaj'^s in-
clude the proximal marginal ridge. In teeth of a squared form
and prominent lingual ridges the marginal ridges should be in-
cluded and the outline carried along the axial slope of the ridge.
The fact that many cases show a lingual articulation and occlusion
on the lingual marginal ridges of upper incisors, will bring de-
mands for including within the cavity the major portion of these
ridges, unless supported by a good bulk of sound dentine. The
failure to recognize this fact on the part of many operators is re-
sponsible for the loss of a large per cent of this class of fillings.
Resistance Form. No special resistance form other than that
just given is required in this class of cavities.
Retention Form. When this order in the preparation has been
reached attention should be directed to the incisal angle, particu-