Page 170 - My FlipBook

138 PREPARATION OF CAVITIES

If the underlying dentin be first undermined by decay or
through the action of cutting instruments, these rods may be

easily chipped away in layers in the direction of their length
by means of chisels. Owing to the fact that they run in
various directions, depending on location, it is necessary that a

brief study of them be here made, in order that we may intelli-
gently and expeditiously chisel them away.

Fig. 171.—Radiation of enamel rods on an Fig. 172.—Proximal view of a cen-
incisor. Labial view. (Diagrammatic.) tral incisor showing radiation of
enamel rods. (Diagrammatic.)

Rule.—On plane surfaces the enamel rods are placed
perpendicular to the dentin; on concave surfaces they radiate

toward each other, while on convex surfaces they radiate
away from each other.
By referring to the illustrations (Figs. 169 and 170), it
will be seen (i) that their outer ends approach each other at
pits and fissures in bicuspids and molars: (2) that they
radiate more and more to the perpendicular, as the points
of the cusps are reached; (3) then, as we go over to the/i