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THE CLEANSING OF CANALS. 459

canals, should not be made so large as to weaken the crown, or there is
danger of fracturing it when in physiological use.
Cavities in bicuspids invading the pulp are usually upon the ap-
proximal surfaces ; they are to be extended over the occlusal face of
the tooth until access to the canals may be had (see Fig. 452).

Fig. 453.
Fig. 450. Fig. 451.
Fig. 452.

Cavity in bicuspid.

The same procedures are to be followed in molar teeth. In lower
molars if the carious cavity be upon the distal wall, it is to be artificially
lengthened across the occlusal face until the probe may be carried
directly into each canal (Fig. 453, a) ; the same method is pursued if
for a mesial cavity. In upper molars, especial care is required to gain
primary access to the anterior buccal root, and tooth structure must be
cut away until this access is secured (Fig. 453, b). Should the carious
cavities open upon the buccal faces of the posterior or lingual faces of
the anterior teeth, the upper cavity edge, that farthest from the gum,
must be extended toward the cutting edge of the tooth until a bent
probe may be readily passed to the apex of each root (Fig. 453, c). In
operating upon many, or most, of the canals of the posterior teeth it is
necessary to bend the pulp extractor or canal cleanser until it is almost
or quite at a right angle with the instrument carrier.
In the six anterior lower teeth where openings are to be made in
them in the absence of large cavities of decay, entrance is effected
through the lingual wall.
The advice of Dr. J. Foster Flagg is appended, as to the position
of tap openings to be made in the several teeth, when the teeth if
carious have not the carious cavity in such position as to afford access,
to the pulp chamber :
" By means of a diamond drill or an inverted cone bur, a rough
spot is made in the centre of the face to be perforated ; this prevents
slipping of the spear-pointed drill which is then employed to enter the
pulp chamber. The outlines of the chamber are to be obliterated with
burs." The dentate bur is a most effective means of enlarging such
openings. " The opening is to be enlarged until a fine probe may be