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94 THE TECHNICAL PKOCEDUKES IN FILLING TEETH.

Histological Chaeacteks of the Enamel in Relation to the
Peepabation of Cavities.

Enamel is composed of rods or fibers cemented together by
an intervening cement substance. These rods and cement sub-
stance have very nearly the same density or quality of refracting
light, 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 sug-
gesting 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 disap-
pear. Still, by working carefully with very dilute acids, the
surface may be so etched as to give excellent views of the enamel
rods.
The enamel rods seem to be made up of globules or little
balls pressed together in rows or lines. One can readily copy
this formation by taking small balls of soft clay and pressing one
upon the other, forming a rod. In some enamel, these globular
forms are very prominent in the apparent make-up of the rod
and produce the cross markings on the rods seen in microscopic
examinations in many of the illustrations. In some other speci-
mens, these globules are so perfectly fused together and so
smooth as to be almost invisible. This latter is rather the excep-
tion than the rule.
The enamel rods are stronger than the cement substance that
unites them together, so that in any attempt to break up or cut
the enamel, it is inclined to split along the length of the rods.
Since the enamel is very hard and difficult to cut, a knowledge of
the direction of the rods becomes of first importance in any
attempt to form cavities in teeth. The rods, while hard to cut,
are, when they are parallel, very easy to split apart. Indeed,
much of human enamel will split almost as easily as straight-
grained pine, if the force is applied in just the right direction to
the margin of any opening that has been previously made. Fig-
ure 100. This is because the cementing substance that cements
the rods together is much weaker than the rods themselves. In