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HISTOLOGICAL REQUIREMENTS IN ENAMEL WALLS. 109

HISTOLOGICAL REQUIREMEXTS FOR STRENGTH IN ENAMEL
WALLS.
1. The enamel must be supjwrted upon sound dentin.
2. The rods whicli form the cavo-surface angle must run uninter-
ruptedly to the dentin and be supported by short rods, with their inner
ends resting on the dentin and their outer ends abutting upon the cavity
wall, where they will be covered in by the filling material.
3. That the cavo-surface angle be cut in such a way as not to expose
the ends of the rods to fracture in condensing the filling material against
them.
The first step, then, in the preparation of an enamel wall is to deter-
mine the direction of the enamel rods by cleavage with a chisel or hatchet.

Fk;. ',n.

Kuaincl hhuuiiii; buth >tiiuUiju ami stiutiliealiuu. (About 80 X.)
In Figs. 92 and 93, No. 1 shoAvs an enamel wall after cleaving the enamel
with a hatchet. It will be noticed that the split has not followed the
direction of the rods exactly, but has broken across them, slivering the
rods as wood slivers in s])litting. This would cause in the cut surface
a whitish, opaque appearance. The plane of the enamel wall should be
extended so as to form a small angle with the plane of the dentin wall,
by shaving the surfiice with a very sharp hand instrument. No. 2 shows
the same wall after it has been extended somewhat ; but it will be seen
that it has not been extended enough, for the rods forming the sur-
face at A do not reach the dentin, but run out at B on the cavity wall,
and that piece would chip out in packing against it or if force came upon
the surface afterward. The angle should be extended so as to produce