Page 59 - My FlipBook

THE ENAMEL.

tion of the enamel rods is parallel to the length of the cusp, or,
in other words, the direction of the rods is perpendicular to the
surface of the tip or point of the cusp.
If from this line around the crown of the molar or bicuspid
in the center of its Length we approach one of the marginal
ridges of the occlusal surface, we will also find the direction of
the enamel rods inclining toward the marginal ridge in such a
way that as we pass over the marginal ridge the direction is so
changed that the rods stand perpendicular to the occlusal
surface.
If the attempt be made to prepare a cavity upon an axial
surface of one of these teeth for filling, and this cavity ap-
proaches a cusp or a. marginal ridge, the enamel wall must be
inclined outward Tor toward the occlusal) very gready, or else
short ends of enamel rods will be left at the surface. And if
inclined enough to prevent this, the filling material will be so
thin at its margin that it will not have sufficient strength.
These facts forbid us laying enamel margins near the
occlusal margins of the axial surfaces of the teeth.
If now, we proceed from the center of the length of the
crown of the molar or bicuspid toward the gingival line, we will
find the direction of the enamel rods again deviating from the
perpendicular and inclining toward the gingival line. This
inclination is much less in extent, however, but more variable
than that found in passing toward the marginal ridges of the
occlusal surfaces. The inclination at the gingival line is usually
about six centigrades, often less, and the greatest that I have
observed has not been more than ten centigrades. An inclina-
tion of six centigrades is readily overcome by inclining the
enamel wall, or even twelve centigrades inclination of this part
of the enamel wall can readily be made and the filling material
be sufficiently strong for the locality.
On examination of the occlusal surface of a molar or bicus-
pid we find a tendency of the enamel rods to incline somewhat
toward the pits and grooves, and especially do they incline
toward open pits or fissured grooves. If the grooves are well
closed and shallow, there will be less inclination toward them.
As we pass from the pits and grooves toward the cusps the
direction of the rods first becomes perpendicular to the surface
and then begins to incline toward the cusps, and when we arrive