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CAVITY PREPARATION. 109

the teeth very flat, the eml)rasiives shallow, and much of tliese
surfaces are in near approach to each other, decay beginning in
the enamel will be likely to spread from angle to angle, as shown
by the whitened enamel in the transverse section of a l)icus])id,
Figure 115. It will be noted particularly that this condition gives
the poorest opportunity for the cleaning of these parts by the
friction of food in mastication.
These conditions furnish the information upon which cut-
ting for extension for prevention should be based in the prepara-
tion of proximal cavities. The most general rule is to extend in
each individual case toward the buccal and lingual angles of the
tooth to such a j^oint that the cavity margin will be so separated
from near contact with the proximating tooth that food will run
between the two in mastication and keep the margin of the filling
clean. The amount of this will, of course, deijend upon the form
of the contact point and of the rounding of surfaces of the teetli.
On the buccal surfaces the area of greatest susceptibility is
usually near the center mesio-distally, close to, but just a little
to the occlusal of the free border of the gum. This is a variai)le
point. In the child the gingiva^ are long, covering a considerable
part of the crowns of the teeth. As the child grows older, the
gingivEe become shorter and cover a less amount of the crowns
of the teeth. As the person grows old, the gingivae disappear
almost entirely so that practically the whole of the enamel of the
teeth is exposed to view. Therefore if decays in this position
have begun in the child, they may be mid-length the crown when
the child has arrived at adult age. Therefore the particular posi-
tion of the beginning of these decays on the buccal surfaces will
depend much on the age of the patient at the time of their begin-
ning. They are, however, always near the free border of the
gingivfe at the time of beginning. If such cavities are filled for
j'oung persons without pushing the gingivae well back in each
case so that the border of the cavity can be extended well under
it, the natural shrinkage of the gingivge will soon expose the
enamel between the gingiva? and the filling. This will then be
liable to decay again. Fortunately these decays are not frequent
in young people.
The extension of these cavities from the center of the sur-
face far toward the mesial and distal angles of the tooth is so
common that the rule should be to cut them close to the angles of
the tooth in every case, but never past the angle. That is the
only safe course to pursue. If that is not done and the suscepti-
bility to decay (which will receive mention below) continues,