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68 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.

CARIES OP DENTIN.
ILLUSTRATIONS: FIGURES 63-74.
Although in the natural order of the phenomena, caries of
enamel always precedes caries of dentin, it is preferred to first
describe caries of dentin for the reason that the order and char-
acter of the processes in its progress are regarded as having
been completely made out, and it is believed that afterward
caries of enamel will be easier understood. It is sufficient here
to say that caries of dentin can not occur until the enamel has
been penetrated. The enamel with its rods cemented together
by its cementing substance is a solid. It has no natural open-
ings into which microorganisms can grow; and these have no
power of penetrating into it, except as it is dissolved and
removed by the acids which they form during their growth.
Therefore, in decay of the enamel, the microorganisms producing
the acid are on its surface. On the other hand, the dentin is
everywhere permeated by the dentinal canals into which micro-
organisms may grow when the dentin is exposed by the destruc-
tion of the enamel.
Both caries of enamel and caries of dentin are caused by
the same agency; namely, the growth of microorganisms in
contact with the surface of the enamel in the first instance, and
the formation of lactic acid during that growth, which dissolves
the calcium salts of which it — the enamel — is composed. In
caries of dentin, the microorganisms grow into the dentinal
tubules and form their acid product within the tissue itself. This
dissolves the calcium salts of the dentin, converting it into the
soluble salt, calcium lactate, which gradually escapes into the
surrounding saliva by osmosis.
Osmosis. The word osmosis, as used here, is used in phys-
ical chemistry to represent the passage of soluble salts through
animal membranes. This occurs whenever a fluid containing the
specified salt is in contact with the membrane upon the one side
and water is in contact with the same membrane upon the other
side. In this case, the salt will pass through the membrane
into the water until the water on either side of the membrane
becomes equally salt. Or again, if two salts, which will commingle
in solution without chemical reaction, are placed in solution in
water separately, and the one placed in contact with one side of
a membrane and the other in contact with the other side of the