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66 PATHOLOGY OF THE HAED TISSUES OF THE TEETH.

tooth, never from within the tooth. It is something extraneous
to the tooth, acting upon the surface in the beginning and pene-
trating little by little into its substance.
Caries of the dentin is different from caries of the enamel,
in that the organic matrix is sufficient in amount and consistence
to retain its histological and physical forms after the solution
and removal of the calcium salts. With the removal of these
by an acid, the enamel will have disappeared entirely, but the
dentin will not suffer any change of form. Therefore, the simple
solution of the calcium salts leaves a softened matrix in the
dentin and does not form a cavity. After the solution of the
calcium salts, there is a decomposition of this organic matrix
progressing from without inward, breaking it up and finally
forming a cavity. Between the solution of the calcium salts and
the decomposition of the organic matrix, some little time passes.
The two seem never to occur at the same time in any given por-
tion of the tooth, but the calcium salts are dissolved out first
and the decomposition of the organic matrix follows later.
Therefore, while, after the full development of the carious proc-
ess, both may be progressing at the same time, the solution of
the calcium salts is always considerably in advance of the decom-
position of the organic matrix, leaving a zone of softened mate-
rial between.
In the progress of the solution of the calcium salts of the
dentin, the tendency is to spread in every direction from the
point of penetration of the enamel, and especially along the
dento-enamel junction. Then, particularly when the original
opening in the enamel has been small, the enlargement of the
opening is brought about mostly by what is termed "backward
decay of the enamel." This is decay of the inner surface of
the enamel that is in contact with the decaying dentin under it,
due to the spreading just mentioned. In this case the enamel
decays from the inside outward until it is so weakened that it
breaks away, enlarging the opening. The rapidity of this back-
ward decay of the enamel is exceedingly variable. Cavities
may be wide open early in their progress, or they may remain
for a considerable time with a small opening.
The calcium salts are not dissolved complete from without
inward in the first instance, but the acid which dissolves them
seems to spread, or percolate, into the tissue and the solution
goes on as a gradual softening process. The innermost part
affected is less softened than the more superficial parts. In the
outer portion of the softening area all, or about all, of the cal-