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PROPHYLACTIC TREATMENT OP THE MOUTH 181

the last tAvo are responsible for most of the destruction of the
hard dental tissues by caries.
Composition of Salivary Calculus. Mixed saliva contains in
man an average of about 0.5 per cent solids. The calculus is pre-
cipitated into the mouth in a form of a finely divided calco-
globulin, Avhich collects in masses upon any stationary object,
close to the mouth of the gland ducts. The fresh deposit is very
soft and greasy to feel when first deposited, but within twenty-
four hours it begins to harden and increases in hardness up to the
time of thirty or sixty days, Avhen it has generally attained its full
hardness and will break aAvay from the stationary object in masses
showing distinct lines of fracture.
Lime Salts Held in Solution. Calcium phosphate and magnesium
phosphate are held in solution in the saliva, made possible by the
presence of a little carbon dioxide.
Reasons for Precipitation. When the saliva is discharged into
the mouth it is released from the normal blood pressure and some
cf the carbon dioxide escapes which allows the calcium salts to be
precipitated. The lactic acid Avhich is continually formed in the
mouth converts the mucus into a curd in Avhich the calcium salts
are entangled to harden into salivary calculus. This process is as-
sisted by the presence of the oxygen taken into the mouth with the
breath, which facilitates the liberation of the carbon dioxide, in
the process of oxidization.
Time of Deposits. It would seem from the experiments of Dr.
Black that the deposits of salivary calculus are paroxysmal and
also that these periods of rapid deposit follow the ingestion of
heavy meals. He thinks that these periods of excessive deposits
come at a time when the blood is overcharged with food pabulum.
Kind of Food. It does not seem from his experiments that the
kind of food has very much to do with these deposits. The more
easily a food is digested, the more quickly following the meal will
these deposits appear.
Habits of Patient. It would seem that the habits of the patient
have little to do in influencing the amount of these deposits.
However those who live a life of physical exertion, Avhich favors
the using of heavy meals have a greater tendency to deposits of
tartar than those whose vocation would cause them to eat lightly.
Mouths Most Subject to the Deposit. From our present under-
standing of this subject it would seem that the mouths most sub-
ject to the deposit of salivary calculus are those indi^aduals,