A further important difference between human and animal milk lies in the fact that according to the present state of our know-ledge, we are forced to administer milk in a boiled or sterilized, at any rate pasteurized, condition, a practice which in most countries should not be changed too soon. Even if we assume that the digestibility of the casein is not influenced by this proceeding, nevertheless the albumins are coagulated, the emulsion of the fat is damaged, the salts are partially freed from their organic compounds, and the power of self protection against bacterial destruction possessed by raw proteid is lost.
Recently, attention has been directed by a number of authors to certain enzymes peculiar to fresh milk (see Marfan). Their sig nificance, in the first enthusiasm of discovery, was greatly exaggerated, but they are nevertheless of fundamental interest, because through them each kind of milk hears to a certain extent its vital label. Whether they play a part in digestion, such as supplementing the undeveloped digestive function of the infant, we do not know, although the results of many clinical experiences and metabolism experiments appear to be explicable by such a theory (Monrad, Hohlfeld, Cronheim-Mfiller). At any rate these substances are not resistant to heat, are destroyed by the usual cooking and sterilizing methods, and are weakened by pasteurization. Hence they constitute another important point of difference between natural and artificial feeding.
The increased immunity attained through breast-feeding finds its explanation in the researches of Salge, who showed that homologous albumin acts as a conveyor of antitoxin from mother to child, and also in the conclusions of Moro, who demonstrated a higher bactericidal power in the blood serum of naturally nourished infants.
Tbe obtaining and preserving of cow's milk, aseptically is so costly and the addition of substances such as salicylic acid, boric acid, for maldehynle, etc., to hinder bInterial development is either so ineffective, or so injurious, that we are left with nothing practical except to aim at relative sterility through boiling or sterilization. As to other methods, such as tile use of ultra-violet rays (Seifert), we must wait for further experience. Does sterilization really accomplish what we expect of it? On the long road between the udder of the milk-giving animal, and the mouth of the infant, are many opportunities for bacterial contami nation. Numberless organisms gain entrance to the milk, f t oin the body
of the animal itself, from its hair, from its tail, from its manure, from the air of the stable, from rubbish, front fodder, from flies, from the hands and clothes of the milkers, from straining cloths and pails, and filially from the various manipulations during transportation within and without the house. These organisms under proper temperature conditions, find an opportunity to increase immeasurably. As appears from the bacterial counts of Miguel, and of Escherich and Cnopf, they make the most thorough use of this opportunity. As a result of a closer investigation of this condition of milk contamination, physicians were brought to conceive it as the decisive cause for the high mortality statistics of artificially nourished infants, and looked to thorough steril ization for the surest means of prophylaxis. There are still at the present time enthusiastic supporters of this view.
Of the contaminating organisms we will for the present disregard the pathogenic varieties which excite specific diseases, and will con sider the saprophytic varieties. These, on account of their different action upon milk, can be divided into two groups, the acid-forming and the proteolytie. The former cause a fermentation of the milk sugar, with the production of volatile acids, and produce a precipi tation of the casein, through which the atilk is coagulated. Such an altered nutriment, even under the most poverty-stricken conditions, can hardly be utilized by the infant. Moreover the fission fungi belong ing in this category are not spore forming organisms, and therefore have little resistance to heat, and are not endowed with the power of toxin formation, so that the usual heating process accomplishes the destruction of their vitality. The only question is, whether the protoplasm of these acid producers contains injurious substances which after the destruction of the germ can pass into the milk, and also whether the administration of numberless dead bacteria of this group can be a cause of danger. We still know very little about this matter, although investigations on animals by Jemma and Figari suggest that morbid symptoms referable to the digestive system may be produced in this way.