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THE ENAMEL, 53

enamel these globular forms are very prominent in the apparent
make-up of the rod, while in some other specimens these globules
are so perfectly fused together and smoothed as to almost dis-
appear ; we then have a smooth enamel rod. This latter is
rather the exception than the rule.
The enamel rods are stronger than the cement sub-
stance, so that in any attempt to break up or cut the enamel it
is inclined to split along the length of the rods. Then, 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. These 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. This is
because the cementing substance that cements the rods together
is much weaker than the rods themselves. In other specimens
of enamel the rods, instead of lying parallel with each other,
are very much interwoven and twisted together ; so much so,
indeed, as to prevent them being readily split apart. This
enamel, instead of splitting like straight-grained pine, is more
like the pine knot, which is very difficult to split or cut, as com-
pared with the straight-grained wood. Formerly it was sup-
posed that this difference in the enamel to cutting instruments
was due to a greater amount of lime salts, or the hardening ele-
ment. We have learned in recent years, however, that this is
not the case. The one contains no greater proportion of lime
salts than the other and will not resist decay any better. But
the difference is due wholly to the difference in the relative inter-
lacing of the enamel rods.
This interlacing or twisted form of enamel is usually
confined to the inner half of its thickness. While in the
outer half of the enamel the rods are parallel and will split apart
readily, the rods of the inner half are interwoven or twisted in
such a way as to prevent splitting. Therefore, when a chisel or
enamel hatchet is applied for the purpose of splitting it off it will,-
if supported by sound dentin, split but about half way, and the
remaining half of the thickness is removed with great difficulty.
It is then almost a necessity that we undermine this by cutting
away the dentin from beneath it, after which we may break it
down f|uite reachly.