[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° 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 upper hand, but in the case of our special soured milk we kill out, practically, all wild bacteria and germs, and the pure culture having the field to itself, we can conduct the operation at a higher temperature where the action of the bacteria is at its maximum, and so obtain the necessary lactification in the minimum of time.