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EXCAVATION OF CAVITIES BY CLASSES. 141

mastication and washings by the saliva. For that reason there
is no tendency for decay beginning in pits or fissures to spread
on the surface of the enameh Therefore, superficially, decay
beginning in these points is strictly confined to them and spreads
only within the substance of the tooth.
In the smooth surface cavities, the conditions are reversed.
The surfaces in which they begin are all smooth, polished and
free from pits or roughness of any kind. But they are all in
positions in which the teeth are not well cleaned by friction or
by washings by the saliva, as between the teeth in the proximal
decays, and near the gums in the buccal and labial decays. The
centers of these unclean areas become the nidi, or special points,
in which colonies of microorganisms lodge and develop ; and
where they remain covered in and protected from washings by
the saliva, forming acid in contact with the enamel. In these
positions they are not confined to the immediate nidus or point
of first beginning of growth, as in the pits and fissures, but
gradually spread over the surface to the limit of the unclean
area, unless stopped by the falling away of the enamel support-
ing the central nidus. When the enamel in the center of the
area or point of attachment is destroyed, the remaining marginal
portions of the superficial colony are usually destroyed also. If
a filling is so placed as to restore the original form and condi-
tions, a new colony will probably become attached and grow on
the filling in this restored locality. The filling being indestructi-
ble, this colony will continue to spread until it has extended as
far as the scouring by mastication will allow. If the filling made
is not broad enough to reach a line where this scouring will pre-
vent the second colony spreading beyond its margins, decay is
liable to begin again beside the filling. This requires that certain
extensions be made in the preparation of smooth-surface cavities,
often cutting away more or less sound tissue at certain points, or
in certain directions, for the purpose of preventing the recur-
rence of decay.
The areas of the surface of the enamel about the angles of
the teeth extending occluso-gingivally are found to be most gen-
erally immune to the beginnings of decay. These angles of the
teeth are the mesio-buccal, disto-buccal, mesio-lingual and disto-
lingual. During mastication, the food is driven through the
embrasures oj^ening to the buccal and to the lingual between the
teeth, tending to keep these lines clean. Therefore, in the prepa-
ration of proximal cavities, the object should be to take advan-
tage of these conditions and lay the buccal and lingual margins