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CAKIES OF THE TEETH. 67
cium salts have been dissolved, while in the inner portion but
very little is yet dissolved. In this way the action of the acid
progresses slowly from without inward.
The decomposition of the organic matrix of the dentin pro-
gresses in a similar way, the tissue being broken up little by
little progressively, from without inward, so that the formation
of a cavity, especially when the opening through the enamel is
small, follows slowly the solution of the calcium salts. There-
fore, it often happens that the solution of the calcium salts has
progressed much more rapidly than the decomposition of the
organic matrix. In that case, we find a very large amount of
material that is soft and spongy, which may be easily cut with
a sharp excavator; or, after removing any overlapping enamel
that may interfere, a spoon excavator may be passed along the
margin of the decayed area, and the whole of it may be turned
out in a body, soft enough to be cut with a razor or sharp knife.
Occasionally we will see almost the entire interior of the enamel
cap, i. e., almost the entire dentin of the crown of the tooth,
softened in this way and yet the organic matrix is not yet broken
up. This, however, is the exception to the rule. The general
rule is that the decomposition of the organic matrix follows
fairly closely the removal of the calcium salts.