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FIFTEENTH LECTURE.

Caries of Dentin.

In my last lecture I called your attention as strongly as I
could to the invasion of the enamel by caries—how it begins
and where it begins. To-day I want to speak of caries of the
dentin especially, and call your attention to the process as it
occurs, in detail. It is sometimes thought by dentists, and by
dental students, that the consideration of dental caries is a mere
matter of scientific investigation that has no practical bearing
upon daily office practice. Let me say to you, gentlemen, that
it is the basis upon; which you must found your future practice;
your methods will depend upon your understanding of the proc-
esses of dental caries, and for this reason you should give the
details of these processes your closest attention.
You will remember that I told you that micro-organisms
do not enter enamel ; it is solid, presenting no natural openings
into which the micro-organisms may grow. Therefore, caries
is begun by the attachment of micro-organisms to the surface
of the enamel, where they grow and produce an acid in position
against the enamel, where it acts to dissolve the calcium salts
and thus destroys the enamel. Until enamel rods have fallen
out, micro-organisms cannot enter it. But when the enamel
has been destroyed and they have gained access to the dentin
by the falling out of the enamel rods they grow into the den-
tinal tubules. The dentin, then, presents natural openings into
which micro-organisms may grow, and they become implanted
within the dentin. In this respect caries of the dentin is dif-
ferent from caries of the enamel ; micro-organisms are not able
to enter the enamel, they are able to enter the dentin. Garies
of the dentin also differs in another important respect from
caries of the enamel. When the lime salts have been dissolved
out from enamel there is no basis substance, or not sufficient
basis substance to hang together, consequently the solu-
tion of the lime salts of enamel breaks down the enamel
completely and forms a cavity. It is not so with dentin. There

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