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64 THE MICRO-ORQANISMS OF THE HUMAN MOUTH. — ;

ments made without a knowledge of the form of the given
bacterium.
Question 2 is to be answered by examining the pure culture
on various nutrient media with the naked eye, or under weak
magnifying power.
Question 3 is most easily answered by placing a thin glass plate
or sheet of mica, on the plate culture ; if the bacterium be aerobic,
it will flourish only till the oxygen in the gelatine is consumed
the colonies will consequently present a stunted appearance. If
it be anaerobic, the colonies under the mica will grow faster
than the uncovered ones. The experiment may also be made
in vessels from which the air has been exhausted, or in w4iich
it has been replaced by hydrogen.
Question 4 is decided by cultures on different media, prin-
cipally on carbohydrates, then on albuminous substances, or in
mixtures of both, as they generally occur in the mouth.
In order to answer question 5, cultures are made («) in amyla-
ceous substances, which, when the culture has obtained its full
growth, are to be tested for sugar ; (/>) in solutions containing
cane-sugar, which are tested for dextrose, levulose, etc., or as to a
change in their rotatory power ; (c) in solutions of albumen, which
are then tested for peptone, or on coagulated albumen, which
liquefies when the bacteria have a peptonizing action. We must
watch the course of the fermentation, its duration, and especially
note whether acids are generated or not, and what they are.
Question 6 is naturally decided by ocular inspection.
Question 7 by the usual disinfecting experiments, some of
which may be found described in full in Chapter IX.
The pathogenesis (question 8) must be determined by experi-
ments on living animals, for which purpose mice, rabbits, and
guinea-pigs are most commonly made use of; less frequently
dogs, fowls, or even sheep, calves, etc.
The material may be introduced into the animal body
1. In form of powder, by inhalation.
2. With the food.
3. By cutaneous inoculations (performed by slightly scratch-
ing the purified skin with a sharp instrument charged with the
material to be tested).