Page 60 - My FlipBook

56 THE TECHNICAL PROCEDURES IN FILLING TEETH.

at the point of the cusps the direction will be perpendicular to
the surface at the immediate point of the cusp. Or, if we follow
the direction of the rods passing from the fossae of the occlusal
surface to the marginal ridges, mesial and distal, we find the
direction of the rods first deviating from the perpendicular
toward the center of the fossae, but as we pass toward the mar-
ginal ridges the deviation is toward the marginal ridges, until,
when we arrive near the crest of the ridge, the direction of the
inclination is decidedly toward the ridge.
Therefore, in the preparation of cavities in the occlusal sur-
faces of these teeth the enamel wall may be cut perpendicular to
the general plane of the occlusal surface, so long as the enamel
margin does not approach too closely to cusps or the marginal
ridges, for a slight bevel of the cavo-surface angles will insure
a solid margin without any short ends of rods. But if the
cusps or the marginal ridges are closely approached the inclina-
tion of the enamel wall should be toward the ridge and the cavo-
surface angle beveled rather more strongly in order to insure the
removal of all short ends of enamel rods, and to secure a firm
margin.
In the incisors and cuspids, if we draw a line around the
crown at the junction of the middle and gingival third, we will
find the enamel rods perpendicular to the surface upon the
mesial, distal and labial surfaces, and generally nearly so upon
the lingual surface. In these teeth the direction of the enamel
rods at the junction of the middle and gingival third of the lin-
gual surface is very variable, because of the differences of the
prominence of the linguo-gingival ridge and the depth of the
grooves of the lingual surface. In cases in which the linguo-
gingival ridge is prominent and the grooves deep, or fissured, '
there will be a strong inclination of the enamel rods toward the
grooves. If, on the other hand, this surface is smooth and even,
with the grooves perfectly closed, so as not to be apparent, the
direction of the enamel rods will be perpendicular to the surface.
As we pass from the junction of the middle and gingival third
of the crown toward the incisal margin of the lingual surface,
the inclination of the enamel rods is more and more toward the
incisal. At the junction of the middle and incisal third the
inclination is from six to twelve centigrades, and in the incisal
third it often is fifteen or eighteen centigrades before the incisal