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776 DENTAL CARIES. ;

out, cannot produce caries. There is no reason why an acid commin-
gled with the fluids of the mouth should exert a localized action on the
teeth. Their action Mould be general, and greatest on the exposed sur-
faces. It is not impossible that some such effect is produced—indeed,
I have seen evidence that such is the case—but this does not seem to-
have any relation to the localized action which represents the inception
of caries.
Acid mucus occurs under circumstances that seem in certain cases to
connect it with the inception of caries. These are those forms of the disease
which occur near the margin of the gum in connection with points of
irritation. In irritations of the gum-tissue there often arises a markedly
acid condition of the mucus exuded at that spot. This has been espe-
cially noted by Tomes and Magitot, and is regarded by them as a direct
cause of decay of the teeth. I cannot, in the light of recent develop-
ments, regard it as a cause of caries proper, but it may be productive
of a softening of the tissue at that point or an actual solution of the
lime salts, which will give the opportunity for the implantation of the
agents productive of fermentation and caries. Certainly, observation
seems to connect local irritations of the gingivse with a certain propor-
tion of that form t)f caries beginning about the necks of the teeth ; and
it is sufficiently demonstrated that under these circumstances the mucus
is markedly acid. It should be noted, however, that in these cases the
irritation is not of such a character as to be productive of pus. All
observation shows that pus, wherever it occurs, is directly preventive
of caries. If under any circumstances pus is continuously discharged
into a cavity of decay, the decay ceases. This furnishes a reason
why roots of teeth protruded through the gum, and thus exjjosed,
so seldom decay. There is generally an irritation of tissue in their
neighborhood sufficient to keep them more or less bathed in pus. This
is also noted in case of roots which have lost their crowns, and the gum
of which is in such a condition as to keep its broken end bathed in pus
also in som6 cases in which chronic alveolar abscess discharges into a
cavity through the pulp-canal. This affords a condition of environ-
ment that is markedly opposed to caries ; and, though pus is dis-
tinctly different from saliva, it is still an evidence of the possibility
—we may say the probability—that changes in the constitution of the
saliva may occur that will render it unfavorable to the propagation of
caries.
There is another form of acid mucus that seems to be distinctly differ-
ent from that ])roduced by irritation of the gums, in that no irritation is
apparent and the condition is more or less permanent. In some cases it
seems to be hereditary. It may, however, be acquired. In this condi-
tion the mucus seems much indisposed to mix with the other fluids of
the mouth. It is more viscid than normal, and may be drawn out in
long threads by touching tlie finger to the gums and withdrawing it, I
have seen cases in wliich these threads could be extended to the arm's
length before they wovdd break. This character of the mucus has been
noted by a considerable number of writers, and seems to have been
regarded as an active cause of caries. It certainly appears to furnish a
condition favorable to the propagation of the disease, for in most of the