Page 145 - My FlipBook

CABIES OP DENTIN. 71
have been dissolved out for some little distance in advance of
them. In the natural order of the advance of decay this is
accomplished in the beginning by the percolation of acid through
the enamel before the enamel rods have fallen away, so as to
admit microorganisms to the dentin. This will be more clearly
shown in considering caries of the enamel. Therefore, when
they are admitted to the dentin, some portion of it has been
softened and the organisms begin growing into the tubules at
once. As they do so the acid which they are forming percolates
into the dentin in advance, dissolving more and more of the
calcium salts. In this way it happens that the dentin is contin-
ually softened in advance of the growing organisms so that
there is a little space softened around them that contains none
of the fungi. By removing all overlapping enamel carefully
to give good opportunity, a thin, sharp spoon excavator may
be passed closely along the hard dentin at the margin of the
softened area and the decayed portion turned out, removing
with it practically every microorganism in the dentin.
As the microorganisms fill the dentinal tubules, the tendency
is to become more and more crowded together and the tubules
begin to be enlarged. In some cases this enlargement is a very
regular increase in size along the length of the tubules, the outer
ends of which are enlarged most, as shown in Figure 73. In
other cases there is much tendency to irregular swellings of the
tubules, as shown in Figure 74. Indeed, these two illustrations
have been chosen as showing the extremes of regularity and of
irregularity in this respect. Every variation between the two
may be found. Also the number of side branches into which
organisms may grow varies indefinitely; after passing a little
way from the dento-enamel junction some cases show very few
or even none at all, while in others they are very plentiful.
This enlargement of the dentinal tubules continues until
the division walls disappear, uniting two in one, three into one,
and so on until there is nothing left but a mass of microorgan-
isms mingled with some undissolved shreds of organic matrix,
which, if the cavity is exposed to the saliva, wastes out and is
washed away. This enlargement of the tubules and destruc-
tion of the organic matrix is accomplished by the enzyme of the
organisms. It is digested. Dr. Miller, in his examination of
this point, decalcified teeth, cut them into slices, completely
removed the acid by which the decalcification was done, and
planted the fungus upon these as culture media. He found that
they make a good growth without other nutrient material.