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84 PATHOLOGY OF THE HAKD TISSUES OF THE TEETH.
lute alcohol, which had been filtered to free it from all insoluble
particles. (4.) It was allowed to lie in an abundance of the
shellac solution for several hours. (5.) It was then placed on
a steel disk and the specimen was clamped down onto the cover-
glass with a heavy weight, the excess of shellac removed and
then left until the shellac had become hard. (6.) The cover-glass
to which the specimen was attached was then cemented to a
grinding-disk, which was placed in the grinding machine and tbe
specimen ground thin enough for examination with a twelfth-
inch immersion lens. (7.) The glass, with the specimen, was
then removed from the grinding-disk and at once mounted upon
an ordinary slide, using for the purpose xylol balsam, which
does not dissolve the shellac by which the section is attached to
the cover-glass. In this way, every part of the frail tissue is
retained in its normal position. From the photomicrograph of
this the engraving is made without the slightest retouching.
It is seen by examination of the illustrations that the
spaces, formed between the enamel rods by the solution of the
cementing substance by the acid causing the injury to the tissue,
have been filled by the shellac, which photographs dark. This
divides the carious area, marked by the letter x, sharply from
the sound tissue, marked by the letter e, and displays the enamel
rods, or the undissolved central portions of them, lying in a
dark field, each in its normal position. The adjacent undecayed
enamel does not absorb the shellac and remains white. In Fig-
ure 96 the sound enamel shows scarcely a trace of structure,
while in Figure 97 the enamel rods in the sound tissue are very
well brought out. This difference in unetched sound enamel
is often met with. In both specimens the area decayed is also
sharply divided from the normal enamel by a broad, dark band,
which occurs in many specimens in which no shellac has been
used, as will be seen in most of the photomicrographs of decayed
enamel in this volume.
If we examine carefully these illustrations as to the com-
parative sizes of the enamel rods in the undecayed and the
decayed areas, it is found that they are much reduced and more
slender in the decayed areas. This may be made out also in Fig-
ure 98, which has not been filled by shellac, but not so clearly.
In the grinding of this latter specimen, the machine was
adjusted to stop when the specimen was one one-thousandth
(1-1,000) of an inch thick. Such specimens are extremely frail.
When the solution of the enamel rods has gone a little further
than shown in these illustrations, the whole structure falls to