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SYSTEMIC CONDITIONS. 115

SYSTEMIC CONDITIONS.
ILLUSTRATIONS: FIGURES 146-158.
In what has been presented thus far, the immediate active
cause of dental caries only has been mentioned, namely, the
acid produced at the spot by growths of microorganisms. This
presentation would be incomplete without the mention of other
factors. While caries of the teeth is the most prevalent disease
known to man, those in dental practice who have not made a
careful study of the teeth of persons who have no need for dental
operations perhaps do not realize how many are immune to
caries of the teeth. Many grow up from childhood and pass on
to old age, without ever having a carious tooth. These persons
are aways to be found, if we look for them.
In the mouths of these persons the same microorganisms
are found growing, and growing as abundantly, as in the mouths
of persons who are very susceptible to caries. In an examination
of the fluids of the mouth in these persons, it is also found that
the saliva is as acid, as shown by tests with litmus paper, as
in persons who have caries of the teeth. Cultures of micro-
organisms from those immune persons have been made over
and over again, and it has been found that these microorganisms
are of the same character and species and produce the same
results in culture media as do microorganisms taken from the
mouths of those who are very susceptible to caries. Some of
these persons have been followed for years, occasional examina-
tions and cultivations being made, so as to leave no possible
doubt as to the general facts. Therefore, there is something
lying over beyond the active growth of microorganisms and acid
formation by them, controlling caries of the teeth. We have,
therefore, persons who are predisposed to caries of the teeth
and persons who are immune to caries of the teeth.
A predisposition to disease is generally considered to be a
condition of the body juices and cells which renders the person
liable to that particular disease. We do not always know in
what that condition consists, but, from the developments thus
far, we know it to be dependent upon some material form or
combination of matter. We do not know this now of all diseases,
but we do know it of some. Bacteriologists and chemists are
succeeding in demonstrating the material nature of the causes of
disease by finding alexins, antitoxins, etc., and the conditions
in which these are produced and in which they are not produced.