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22 PATHOLOGY OF THE HARD TISSUES OP THE TEETH.
rendering thern useless for this purpose. Figures 27, 28 and 29,
with their descriptions, serve, however, to illustrate the condi-
tion of the tissue injuries fairly well, though neither of them are
of the severer forms of atrophy. Figure 27 particularly shows
the line of interglobular spaces in the dentin extending across
the occlusal surface and dipping down gingivally along the axial
portions of the dento-enamel junction. This figure also shows
well the shortening of the crown between the pulp of the tooth
and the occlusal surface. From the position of these zones of
injury, it will be realized, from a careful study of the lines of
the injury to the dentin, that the total shortening of the crown
of the tooth is between the pulp and the occlusal surface in the
molars. It is also between the pulp and the incisal edge, where
it occurs in the incisors and cuspids. This is the reason that
pulps are so frequently exposed in the preparation of cavities
in these teeth. Dentists generally have not understood that
the pulps were so close to the surface. When it is realized that
these sheets of interglobular spaces, of which the zone of injury
in the dentin showing in Figure 27 and in Figure 29 is a section
spread through the entire area of the crown of the tooth, form-
ing openings through which microorganisms may readily grow,
it will be understood why it is so difficult to prevent the destruc-
tion of these teeth by caries. It will also be understood how
decay may quickly undermine the entire enamel cap, allowing
it to fall away, exposing a blackened stump of dentin in all of
the central portion with jagged enamel upon its margins, with
decay persisting around the circumference where the line, or
sheet, of interglobular spaces dips to the gingival near the dento-
enamel junction. In the child, one is prohibited from forming
a cavity of such depth as required to hold such a broad filling
because of the nearness of the pulp of the tooth.
The grave significance of this lies in the fact that caries
beginning in the imperfections of the occlusal surface quickly
reaches this zone of interglobular spaces and spreads rapidly
through it, undermining and destroying the whole occlusal por-
tion of the tooth. It often happens that the whole of this has
been swept away so quickly that the area is uncovered before
decay has proceeded further toward the pulp. The decaying
area is then fully exposed to the fluids of the mouth, and the
progress of the caries is arrested. In this case, the tooth remains
as a blackened stump that soon rises in its alveolus and occludes
with its fellow, which is generally in a similar condition, and
both do good service. More often, however, decay continues
in that portion of the zone of injury referred to, that dips under
the strong enamel around the margins of the crown. Being thus