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EXCAVATION OF CAVITIES BY CLASSES. 129

rounding walls and margins should be gone over with a cutting
instrument and scraped to form fresh cut, clean surfaces. After
this, the toilet proper should be made as previously stated.
If a leak in the rubber dam has occurred, or if from
(3.)
any cause some portion of a cavity wall has become wet with
saliva or any fluid exuded from about the gingival margin, or
elsewhere, this should be wiped away with absorbent cotton and
then dried by the air blast. Then all parts noticed as having
been wet, and as much more as may be necessary for certainty
of including all, should be trimmed, planed or scraped, to give a
perfect fresh-cut surface. Then the toilet should be made. In
such cases, the walls of the cavity can not be cleaned by drying.
It is just as important to have cavity walls clean as to have them
dry. The saliva or serum that may exude from the soft tissues
are each loaded with salts, mucin or albuminoids, that are left
on the cavity wall after it has been dried. Figure 127 is a photo-
micrograph of salts from the saliva left from the evaporation of
the distilled water into which these salts were dialyzed to free
them from the mucin, albuminoid and gummy material. The
illustration includes only the margin of salts deposited from a
tiny drop. Any one may see this deposit from the saliva by
placing a small drop of saliva on a clean glass and allowing it
-'"'''
to dry. ' ' '^1^
In the drying of a spot of moisture on a cavity wall, the
mucin and gummy material would be left as a hard transparent
film with the salt crystals. In drying with hot air or cold air
without other preparation, this mass of crystals and gummy
remains would be left and the filling placed over it. These mate-
rials will subsequently dissolve out, forming a leak. This may
not be much, but where these dissolve out, acids may and will go
in if it is at any point on the margin where microorganisms may
grow; and decay may easily recur. If, at a point that is well
cleaned, perhaps it will be seen only as a discoloration, or
so-called blue margin.
(4.) In cases in which the cavity has been prepared and
the filling is delayed until another sitting; immediately before
the filling is placed, the walls and margins should always be so
trimmed as to give perfectly fresh cut surfaces to fill against,
no matter how complete the previous preparation nor how per-
fect the temporary filling may have been. Then the final toilet
of the cavity may be made. In case it is expected that the filling
will be made at another sitting, the final finish of the cavity
should be made at that time. To sum this all up in one sentence,
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