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CABIES OF ENAMEL. 83
faces are of much less extent than in these three figures. Indeed,
the general rule is that the enamel rods have fallen away in the
central area quite a little before the spreading has reached its
bucco-lingual limit. In this case, the spread of decay in the
dentin, along the dento-enamel junction, backward decay of the
enamel and breaking away of enamel, so change the conditions
as to stop the superficial beginnings in the enamel. Figure 95
sufficiently illustrates this. In this case, the spreading bucco-
lingually is not yet great, but the decay has penetrated the
enamel at two minute points, and solution of the calcium salts
of the dentin has begun. In a little more time the enamel rods
would have fallen away from all of the central part of the area
and so changed the conditions that the superficial spreading of
caries would have ceased.
A Closer Examination of the Injury to the Enamel.
ILLUSTRATIONS: FIGURES 96-98.
Before studying further the cause of the localization and
tendency to the spreading of caries in particular directions on
the surface of enamel, it may be well to examine more closely
the injury produced. For the illustration of this, the photo-
micrographs have been taken from thin sections of decays begin-
ning in the enamel that were in every way similar to those shown
in other illustrations, but with an amplification that is sufficient
to display the condition of the tissue without being excessively
magnified. This enables a larger area of tissue injured, in com-
parison with the uninjured tissue, to be shown on the ordinary
book page than seems practicable with a higher amplification.
In Figures 96, 97, special preparation of the tissue has been made
to show the removal of the cementing substance from between
the enamel rods by filling the spaces left with common yellow
shellac, which takes dark in the photomicrograph. Figure 98
is without this. In Figures 96, 97, which are both from cross
sections, but from different teeth, the darker portion marked
with the letter x is the injured enamel, while that portion marked
by the letter e is uninjured. In each case the border line between
the injured and uninjured tissue is dark. The sections from
which they were made were prepared as follows : (1.) The cross
section of the tooth was ground flat and polished. (2.) It was
then placed in absolute alcohol for twenty-four hours to remove
all traces of water. (3.) It was placed on a cover-glass which
was covered with a solution of ordinary yellow shellac in abso-