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micro-organisms. This could hardly be otherwise, for this acid
is disposed to percolate into the tissue, consequently it will be
g-oing before continually as it is formed by the micro-or-
ganisms in their process of growth. Hence, it occurs that we
never find micro-organisms invading the tissue before the solu-
tion of the lime salts. We do not know of any other reason why
this should be so, only the fact that the acid formed will per-
colate in advance of the micro-organisms.
As the inicro-orgaiiisms grow intO' the tubules the tubules
become enlarged. This fact was first noticed by Sir John
Tomes about 1847, I think, and before we knew anything of
micro-organisms. He pointed out that enlargem!ent of the den-
tinal tubules as a distinction that occurred between caries and
the solution of the lime salts by acids, and some years later he
also pointed out that these enlarged tubules were filled with a
granular matter, the character of which was unknown. When

Miles and Underwood, about 1880, succeeded in staining cari-
ous dentin by the methods that had been devised by Koch, they
discovered that this granular substance was micro-organisms.
It had been surmised by many others before that it was micro-
organisms many observers had before conceded that it must
;
be micro-organisms, but Miles and Underwood stained the
micro-organisms in position and got photographs of them in
position that were as clear as this diagrammatic picture (indicat-
ing). This enlargement of the tubules goes on steadily. If the
tubules were of this size at this point (indicating), as the in-
vasion goes on they would soon become enlarged to this size
shown here (indicating), and this enlargement will increase
continuously, the micro-organisms multiplying and crowding
until they are filled to overflowing, the tubules enlarging, ap-
parently, to accommodate them, so that we will find tubules
very much enlarged near the dento-enamel junction, or near
the surface of a cavity that is being formed, while in the depths
they will be of normal size, perhaps, and a little beyond that
there will be no micro-organisms. This enlargemlent of the
tubules goes on until one tubule breaks into another, and the
dividing septi between the tubules disappear gradually, until
finally there is only a mass of micro-organisms and shreds of
imperfectly dissolved tissue remaining; this disintegrates and
washes away and we have a cavity. This is the manner of the
formation of cavities in dentin.

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