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SIMPLE CAVITIES ON EXPOSED SURFACES. 225

lower first bicuspid differs normally from all others of its kind in
having no sulcus and consequently no fissure between the cusps. In-
stead of the two cusps being separated by a sulcus they are united by
a ridge of enamel. (See Chap. I., p. 35.) The only points, therefore,
that invite decay upon the occlusal surface of this tooth are the two
pits that are found one on each side of the ridge. These are to be
filled separately. They probably represent the very simplest form of
simple cavities to be found anvAyhere in teeth.
The occlusal surface of an upper first or second molar presents two
points liable to decay. One is a pit formed by the junction of two
small fissures near the mesial margin, and the other is a fissure which
runs between the disto-buccal, disto-lingual, and mesio-lingual cusps.
Both are represented in Fig. 174. When limited in extent they should
be opened in the same manner as a bicuspid fissure cavity, but when
larger they may be opened by means of a chisel followed by a suitable
bur. In these, as in all cavities in sulci, the fissures must be followed
and opened up to their extremest limits in order to ensure success, while
the margins and marj^inal edges must be so formed as to be strong;,
smooth, and bevelled.
The general form of these cavities when prepared is shown in
Fig. 175. It will frequently be found that these two occlusal cavities
Fig. 174. Fig. 175. Fig. 176. Fig. 177.

Molar fissure cavities. Molar fissure cavities prepared for filling.

are joined underneath, while near the surface they are separated by a
ridge of enamel and dentin. In such cases the ridge should be cut
away and the two cavities converted into a single larger one as illus-
trated in Fig. 176.
If the ridge were allowed to remain it would almost certainly be
fractured either in the operation of filling or subsequently by the force
of mastication.
The upper third molar differs from those anterior to it in having
but three cusps and consequently but one central pit with radiating
fissures. A cavity occurring here when properly prepared will pre-
sent a triangular outline with rounded angles, as in Fig. 177. The
terminals of fissures should always be finally finished Avith a round bur
to prevent any possible angles and opportunity for leakage at those points.
The lower first molar, as well as the third, having five cusps with
intervening sulci, a cavity ujion this surface will be pentagonal in out-
line, as represented in Fig. 178.
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