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PROXIMAL CAVITIES IN INCISORS INVOLVING ANGLE 79
which the fillmg would most likely pivot to exit the fulcniui. By a
study of the case we find we must deal with the force of levers of
both the first and second class.
In Fig. 37 Ave have an illustration of a Class Four, plan one filling
wherein the principles of a lever of the second class are fully opera-
tive. The heavy long lines a-h represent the full length of the
lever. The short heavy lines a-c represent that part of the lever
which is the working arm, as the load is at c. That we may study
tiie amount of anchorage to be provided for at the incisal angle, (c),
we will ignore the assistance of the two gingival point angles and
for that reason they have not been shown in the drawing. We here

Fig. 37.—Drawings to illustrate the principle of the lever in the dislodgement of fillings of the
fourth class, plan one.

have a lever of the second class with the fulcrum at a, the load at
c and the force at J).
In order that we may not inject into the problem at this time the
principle of the bent lever we w'ill consider that by the lateral move-
ment of the mandible the force is applied at right angles to the
"lever-arm." In diagram A, Fig. 37, the working arm is one-half
of the lever Avhich is of the second class. We then have the follow-
ing W'ith X representing the load, or unknoAvn quantity:
100 lbs. 2 : 4 = 400 = 200 lbs. = x.
2x
It would therefore follow that an incisal point angle placed mid-