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140 THE TECHNICAX, PKOCEDTJEES IN FILLING TEETH.

be used in squaring up the axial and surrounding walls, because
tlie angle of approach is such that the sharp angle of the bur is
presented toward the pulp instead of its square end. This neces-
sarily defeats the object as to squaring up the axial and sur-
rounding walls and rendering the axial line angles sharp, and
especially endangers the pulp. All of this work should be done
with the hoes and the smaller chisels.
Except in the smaller cavities, the direction of the enamel
rods will be much inclined to the incisal upon the incisal wall of
the cavity. This should be carefully noted in chipping away the
enamel, and the finished enamel wall should have the proper
inclination to give it the necessary strength.
These cavities not infrequently occur in children soon after
these teeth take their places in the arch. They are then difficult
of treatment, because the long, heavy, free margin of gum on the
lingual is in the way of the proper adjustment of the rubber dam.
This, however, is the only special difficulty. These cavities are
so very dangerous when neglected that a special effort should
be made to fill them while very small.
Smooth-Surface Cavities.
Classes. General Considerations. Smooth-surface cavi-
ties include four classes, as follows : Class 2. Proximal cavities
in bicuspids and molars. Class 3. Proximal cavities in incisors
and cuspids which do not require the restoration of the incisal
angle. Class 4. Proximal cavities in incisors which do require
the restoration of the incisal angle. Class 5. Buccal and labial
gingival third cavities. These comprise by far the greater num-
ber of decays that occur in the human teeth. Occasional smooth-
surface cavities occur on lingual surfaces and at other points,
but they are so infrequent, and their treatment is so similar to
those classified above, that they may be ignored in this connec-
tion.
The smooth-surface cavities are quite distinct in the condi-
tions of occurrence from the pit and fissure cavities. In all pit
and fissure cavities there are actual pits or fissures which cause
the localization of decay in those points. The pits act as shelters
for the growth of colonies of microorganisms in which they may
easily be covered in. In this position their acid products are
protected from dissipation in the general fluids of the mouth.
Therefore the acids are applied directly to the solution of the
calcium salts of which the enamel about them is composed. The
enamel immediately outside the pit is exposed to the friction of