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96 PATHOLOGY OP THE HAKD TISSUES OF THE TEETH.
the cause of the hyperemia of the pulp that so often occurs in
carious teeth before the pulp has become involved or exposed
to the actual carious process.
In studying the decays of this tooth, Figure 107, or others
of this group, it will be seen that any effort to prepare these
smaller cavities as simple proximal cavities, would inevitably
produce conditions which would cause failure of the fillings,
for the reason that they are situated so close to the marginal
ridges that the enamel rods slope very much toward the ridge.
Further cutting in that direction to obtain a clean enamel wall,
or to remove the decay projecting along the dento-enamel junc-
tion, would bring the margin of the filling to an impossible point,
because of the direction of the enamel rods. Therefore, the
marginal ridge should be cut through and anchorage made in
the form of a step in the occlusal surface.
This is more sharply illustrated in the undermining of the
distal marginal ridge in Figure 108, in which decay has made
a little further progress. In such positions, this undermining
is generally done very early in the progress of the decay and
the marginal ridge breaks away, exposing the cavity corre-
spondingly early. This generally gives the patient the first sug-
gestion of the presence of a cavity. Often, also, the dentist
overlooks these decays until this breakage reveals them. This
is often fatal to the pulp of the tooth, especially in cases in
which the decay has begun farther to the gingival, as in the decay
on the left, in Figure 110, or in those cases in which the enamel
cap is unusually strong, as in Figure 111. In the decay seen in
the right of Figure 110, the undermining of the enamel seems
extreme. This is often the case in those decays that have pene-
trated the enamel as a small opening without much superficial
spreading, as is apparent in this case. In Figure 109 there is
an excellent showing of backward decay of enamel in the decay
seen on the left of the figure. In this case the cut is to one side
— the buccal side — of the central area of the decay, and shows
the projection of the decay buccally, along the dento-enamel
junction, its progress in the dentin and the whitened area show-
ing the backward decay of the enamel. Incidentally this cut
has been in the line of the projection of the mesio-buccal horn
of the pulp, which is unusually long, demonstrating again the
clinical necessity of avoiding the recessional line of this horn of
the pulp in the preparation of cavities, both proximal and
occlusal. For here it may also be noticed how very close the
small occlusal cavity is to this horn of the pulp. Finally, it is