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24 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
the spiculsE of cusps will stand much closer together than the
cusps of the normal tooth. The whole of the occlusal surface
of the tooth is dwarfed, often discolored and sunken into the
tooth crown. The pulp chamber will be nearer to the occlusal
surface than in the normally developed tooth in proportion to
the sinking of this part into the body of the crown. Figure 30.
All around this the enamel of second formation wells out in the
form of a broad collar to the normal size of the tooth crown at
or about the base of what would have been the normally devel-
oped cusps. The whole of the injured area is apt to be rough
and pitted, and the pits and grooves are very apt to be wide open.
This condition gives unusual opportunity for the beginning of
caries and the early exposure of the pulp. It is for this reason
particularly that they are so often destroyed very early. In
cases occurring a little later, but before the completion of the
occlusal surface, the conditions inviting the beginning of caries
are equally bad. The cusps will stand further apart, are
stronger, but the central part of the occlusal surface and the
outer slopes of the cusps are in very bad shape. At a still later
date, after the completion of the occlusal surface, the injury
appears as a groove around the crown of the tooth and is gen-
erally of less consequence.
In all of these cases the zone of injury in the dentin is one
of the very grave features, for the reason that caries reaching
this zone of interglobular spaces spreads through it quickly. It
has been exceedingly difficult to get material for the illustration
of this for the reason that very generallj' the occlusal surfaces
of these teeth are destroyed by caries before their removal,
rendering them useless for this purpose. Figures 31, 32 and 33,
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 31 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 the injuries in
atrophy, 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 betAveen 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