e for the
preparation of soured milk, and might, indeed, make it
directly injurious.]

[Illustration: FIG. 39.--Photo-micrograph of culture of
_Bacillus bulgaricus_ in malt. In cases of acute
enteritis, where milk cannot be supported, the lactic
bacilli may be cultivated in malt, and administered to the
patient in this form, with occasional doses of syrup of
malt, in order to induce a vigorous growth of the lactic
ferments in the body.]

[Illustration: FIG. 40.--Photo-micrograph of smear from
milk that had been allowed to sour spontaneously. Ordinary
market milk is always subject to infection from the air,
milking vessels, manure, etc., and from these sources a
mixed bacterial flora arises, with the result that the
milk exhibits curdling, acidity, gaseous fermentation, or
mould growth, after being stored for a short time. This is
due to the action of sush bacilli, diplococci, yeasts, and
moulds as are shown in this illustration.]

However fresh the milk may be, it will contain useless or injurious
bacteria, and we must get rid of these before attempting to introduce
those whose growth we wish to encourage. This is effected by heat. All
the living bacteria and most of the germs are killed at temperatures
somewhat under the boiling point. Having sterilised the milk in this
way, it is necessary, as far as possible, to prevent the entrance of
fresh germs from the atmosphere, and we therefore let the milk cool down
in covered dishes. When the temperature descends to about 100 deg. F. the
culture of the special bacteria is introduced, the covers are replaced,
and the milk vessels maintained at or near this temperature for twelve
hours, when the soured milk is ready for use. It is not necessary to use
fresh culture every time--a little of the soured milk will take its
place, and this may be repeated as many as fourteen times before it is
necessary to start off again with a fresh culture. A great deal depends
on the care exercised and the freedom of the surroundings from bacterial
germs. Under the best conditions wild germs will gradually accumulate in
the soured milk, but their increase may be greatly delayed by attention
to the precautions mentioned. The ordinary souring of milk for butter
and cheese making is conducted in cool surroundings, as already stated,
because in such conditions the lactic-acid-producing germs increase
relatively faster than the wild germs, and so gain the uppe