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52 THE TECHNICAL PROCEDURES IN FILLING TEETH.

intimate knowledge of it that is necessary to the most intelhgent
practice in filling operations.
The enamel when examined macroscopically appears as a
very hard, vitreous body, white, or a bluish-white, very dense
and brittle, in which no traces of structure can be determined.
It cuts with much difficulty, and is much inclined to chip and
crumble. If, however, it is examined with a good hand magni-
fying glass, certain striations can be observed that give a sugges-
tion of histological structure.
Although the enamel seems to be opaque, or, at most, trans-
lucent, by ordinary examination, it is found to be almost as trans-
parent as glass when ground into thin sections. When so
prepared, very little of the structure can be seen with the micro-
scope usually, without some preparation that will cause its histo-
logical elements to appear. It is largely for this reason that so
little is seen of the structure of enamel in the sections ordinarily
prepared for microscopic observations.
Histological Characters of the Enamel in Relation to
the Preparation of Cavities.
Enamel is composed of rods or fibers cemented
together by an intervening cement substance. These rods
and cement substance are very nearly of the same density, so that
when examined in the perfect state the enamel seems to be almost
homogeneous, or without special structure. In the most perfect
specimens of enamel only a striation suggesting structure can be
seen. It has been learned, however, that the cement substance
between the rods by which they are united dissolves more readily
in acids than the rods themselves. We may avail ourselves of
this fact, and partially isolate the rods by solution of the cement
substance with very dilute hydrochloric or lactic acid, and in that
way obtain good views of them. We can not, however, carry
this solution very far, for the reason that the rods will also be
dissolved, and the whole tissue disappear. Still, by working
carefully with very dilute acids, good fragments of the rods may
be obtained.
The enamel rods seem to be made up of globules or little
balls pressed together in a single row or line, forming the rod.
One can readily copy this formation by taking small balls of soft
clay and pressing one upon the other, forming a rod. In some