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its after progress. Then there are certain portions of the
tooth's surface that are Hable to caries which we must learn
to know as areas of liability to dental caries. Caries is pro-
duced by an agent acting from without, an agent that must
have time to grow and to act at some particular point, and
in the process of mastication all of those surfaces of the tooth
that are worn or kept clean by the motions of the lips, cheeks
and tongue, and by the excursions of food over the surfaces,
are protected from decay. The beginning of decay is then
confined to certain areas. For instance, the pits, fissures
and grooves that are so deep as to receive and hold lodg-
ments will not be kept clean by the efforts of mastication,
consequently colonies of micro-organisms will grow in these
positions, form their acids in contact with the enamel and
perform its solution, and the beginning of dental caries will
be manifested. It requires some such thing as this for the
beginning of decay. This, then, is the first position in which
w^e have areas of liability—in the pits, grooves and fissures in
the occlusal surfaces of molars and bicuspids and upon the
lingual surfaces of the lateral incisors, and occasionally the
central incisors also. The other positions of the beginnings
of decay are upon the proximate surfaces—the second posi-
tion—and confined very strictly to the proximate surfaces.
Now, it isn't necessarily the mesial or the distal surface, be-
cause if a tooth is turned so that its buccal surface becomes a
proximate surface decay will begin upon that buccal surface
as if it were the mesial surface. It has no reference to the
surface of the tooth, but has reference particularly to the por-
tion of the tooth approximating an adjacent tooth, closing in
a space upon which lodgments may occur and remain. That
is the point particularly with reference to the beginning of
caries. And the liability to decay will be in proportion to the
freedom from removal of the lodgments which occur upon
the given area. The areas of liability will be large or small in
proportion as the area of the tooth from which lodgments
are removed with difficulty is large or small. Consequently
we will find upon proximate surfaces a central point of great-
est liability and an area of lesser liability, grading away from
that as the surface may be kept partially or completely clean.
This is the second area of liability, and I \till make but three.
The next is labial and buccal surfaces. iVs the tooth pro-
trudes through the gums at first the gum line will be near the

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