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THE HISTOLOGICAL STBUCTUEE OF THE TEETH. 95

other specimens of enamel, the rods, instead of lying parallel
with each other, are very much interwoven and twisted together,
as shown in Figure 101 ; 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 Imot, which is very
difficult to split or cut as compared with the straight-grained
wood. Formerly it was supposed that this difference in the
enamel to cutting instruments was due to a greater amount of
calcium salts or the hardening element. 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 interlacing of the enamel rods.
This interlacing or twisted form of enamel is usually con-
fined to the inner half or two-thirds of its thickness. While in
the outer portion of the enamel, the rods are parallel and will
split apart readily, the rods of the inner portion are interwoven
or twisted in such a way as to prevent splitting, as shown in
Figures 101, 102. Therefore, when a chisel or enamel hatchet
is applied for the purpose of splitting it otf, it will, if supported
by sound dentin, split but part way and the remaining part of
the thickness can be removed with great difficulty. It is then
almost a necessity that we undermine this hj cutting away the
dentin from beneath it, after which we may break it down quite
readily.
The enamel rods are almost always parallel with each other
in the outer part or near the surface of the enamel. This fact
should be observed carefully, for it is of the utmost importance
in the preparation of the cavo-surface angles of cavities. All
that we have said of the splitting apart of the enamel rods,
applies with all of its force to this outer portion of the enamel,
no matter how much the rods may be twisted in the deeper por-
tions, and for this reason cavities should be so prepared that no
portion of the cavo-surface angle will present short ends or rods
unsupported ; that is, if the enamel wall should be parallel with
the length of the rods, a bevel of the cavo-surface angle should
cut off the ends, as shown at a in Figure 10,3, so that there may
be no loose ends of rods upon the surface to fall away while pack-
ing gold or after finishing the filling. This may readily be accom-
plished if we know the direction of the enamel rods. An enamel
wall sloped as shown at b, Figure 103, makes the strongest wall
possible, because there are no free ends of enamel rods in any
part of its surface. In this case, the slope or inclination of the