Page 242 - My FlipBook

122 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
were published was what might be expected under the circum-
stances. They were regarded as revolutionary, and rightly, for
the results proved that many of the ideas of dental caries that
had existed before were necessarily wrong, and other opinions
must take the place of those which had been regarded as correct.
Many arguments were advanced from time to time in the effort
to sustain the older view, most of which have disappeared.
The hardness and softness of the teeth to cutting instru-
ments has troubled a considerable number of men. It has been
difficult for them to feel that all teeth were of equal hardness,
or nearly so, as had been represented. A further careful study
of the subject shows that this has generally been due to differ-
ences found in cutting the enamel of different teeth, rather than
in cutting the dentin. With the exception of its tubules running
through it, the dentin seems to be very homogeneous in struc-
ture. There is no disposition to split in any one direction
more readily than another. Generally no accretion lines are
discoverable, though these may often be brought in view by
decalcification. We can not find them in the calcified dentin with
instruments. In careful experimental work in cutting dentin, I
have been unable to distinguish differences that seem to be of con-
sequence. In the crushing strength, quite a little difference has
been noted, which will be found in the tables of the strength of
dentin accompanying the studies to which I have referred.
In the enamel there are wider differences in the apparent
hardness to cutting instruments. These are due (1) to the direc-
tion of approach, (2) and to differences in the relation of the
enamel rods to each other. The enamel is not a homogeneous
structure like the dentin, but is composed of the enamel rods
cemented together by a cementing substance which is less strong
than the rods themselves and allows them to part on the line of
their length more easily than in other directions. "When the
rods lie parallel with each other, the enamel splits in their direc-
tion easily when a breach has been made and a sharp chisel is
used in the right direction on the margin of the breach. If we
undertake to cut it in other directions with steel instruments,
it is found to be very hard. This is called straight enamel.
Figure 146 is a photomicrograph of straight enamel in which the
rods lie parallel and are straight from the dento-enamel junc-
tion to the surface of the enamel. This enamel with straight
parallel rods is found on very many teeth; perhaps in the
majority of cases we will find the enamel rods straight and
parallel, except over the cutting edges of the incisors and the