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462 THE TREATMENT AND FILLING OF BOOT CANALS.

finest size of Donaldson canal cleanser is passed as far as it will go into
the canal, the cleanser is inserted and partially withdrawn, scraping
away the calcinm sulfate formed by the action of the acid upon the cal-
cium salts of the tooth. The acid is quickly neutralized and fresh
applications are made drop by drop, the scraping ;ind pumping with the
cleanser being contiuued until the point of the instrument is felt to
reach or pass the aj)i('al foramen. Any organic matter, such as filaments
or minute fragments of pulj) tissue, which may luive been present in the
canal is destroyed. This applies also to organic matter undergoing de-
composition or to organisms which may be present. As there is no
marked degree of force required in the operation it may be pursued
even in cases of pericementitis or acute abscess, to gain direct and free
entrance to the seat of morbid action, the focus of germ development.
In the event of the operator being unable to detect through instru-
mental means the openings of minute canals. Dr. Callahan advises that
a pellet of cotton containing a minute portion of acid be placed over the
]n-obal)le situation of each canal and sealed in over night. The follow-
ing day, when the rubber dam is applied and the cavity dried, the spot
of application of acid will be represented by a small white area, in
wdiich, if a canal entrance exist, it will be represented by a black dot.
A pit is made at this point and acid is applied, when entrance by cleansers
is attempted ; should failure to gain entrance result, it is most probable
that the canal is almost or quite obliterated with secondary deposits
formed by a receding pulp, hence no future sepsis is probable. As
soon as the cleanser is felt to touch or pass the apical foramen the
canals are syringed out with a saturated solution of sodium bicarbonate.
Carbon dioxid is disengaged, which drives the debris left in the canals
into the pulp chamber, and the acid is neutralized.
Thus far has been described the entrance to and thorough cleansing
and uniform enlarging of canals of a tooth from which the intentionally
devitalized pulp has been extracted ; the immediate question is, What
treatment shall now be pursued ? Owing to the method of pulp with-
drawal, the contents of the dentinal tubules are as yet chemically un-
changed ; and it scarcely requires argument to demonstrate that, can
they be kept in a stable condition, they constitute the best material for
occupancy of the tubules. Examining the list of medicaments applica-
ble as preservatives zinc chlorid is the agent fixed upon as the one
which will best procure an unchangeable condition of the contents of
the tubules. The experiments of Prof. Jas. Truman ^ have shown that
this agent quickly diffuses through a capillary tube containing albumin,
converting it into a whitish coagulum, an albuminate of zinc, which
every anatomist knows to be one of the most efficient of all preserva-

^ Proc. Academy of Stomatolor/y, Philadelphia, 1894.