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PROXIMAL CAVITIKS IX IXCISORS INVOLVING ANGLE 83

angle to exit. In the fii-st instance we lengthened the axial wall, us-
ing the same length of step. In the second instance we shortened
the axial wall and at the same time lengthened the step and the
change is more rapid. It W'Ould seem then that the direction to be
given the incisal point angle is determined by the degree of the
circle in Avhich lays a line draw'n from the deepest portion of the
incisal point angle to the fulcrum. (See dotted lines Fig. 39.) The
nearer this line in a given case approaches the perpendicular to the
axial part of the filling the more essential is it that the point angle
be cut in the same plane as the axial wall. Also the nearer this
line approaches ninety degrees from the perpendicular the more es-
sential is it that the incisal point angle be cut at forty-five degrees
to the perpendicular of the axial Avail.
Bv a studv of Fig. 40 it will be seen that the incisal angle of

Fig. 40.—Drawinsts to illustrate the importance which should be given to the proper plac-
ing of the incisal point angle in fillings of Class Four, plan two, with particular reference to
the plane in which wall b— c should be cut.
A would be effective while B would offer no resistance to exit with
a filling pivoting at a. By materially shortening the axial walls of
both, the point angle of B becomes effective and that of A ineffective.
As shown in the drawings in A the dentine included in h, c, d is
the retention produced by having dotted line a, h longer than line
a, c. In B these lines are the same length, hence no retention. The
filling becomes a lever to lift the gingival point angles.
The Gingival Angles. In the study of the gingival angle reten-
tion, we will eliminate the incisal angle and consider that it has been
improperly laid or has been Aveakened and the lever force has been
transmitted to the gingival angles.