There are ruminant, human, asinine, and equine varieties of Milk, all available for our sustenance, and curative uses. The essential difference between human milk, and that of ruminant animals (cow, goat, and sheep) lies in the character of their casein, or curd, and its proportions to those of the other albumins. The milks of all mammals consist of water, holding, in virtual or actual solution, salts, sugar, casein, and other albumins, with minute globules of fat uniformly suspended in the liquid, but tending to come to the top by reason of their lower specific gravity. Milk shares with oysters the advantage of containing within itself representatives of all three nutritive main divisions of food, the proteids, the carbohydrates, and the fats. - But because milk is the pattern natural food of the human species, and of mammals, during the whole period of their most rapid young growth, it must not therefore be taken as a model diet for adults. This is evident from the fact that grass is the superlative food for the cow, by instinct, and milk that of the calf.

Human Milk

Human Milk is always alkaline, cows' milk being either alkaline or acid, whilst that of carnivorous animals is acid. Cow's milk contains a little carbonate of sodium, and if a small quantity of acid be added thereto (for instance lemon juice or vinegar), some carbonic acid gas is given off, the sugar of milk being converted into lactic acid, so that then the milk does not go sour. Milk sugar is chemically lactose. Sydney Smith writing (1820) to Mrs. Meynell about the nurture of her infant son, said, "The usual establishment for an eldest landed baby is two wet nurses, two dry ditto, two aunts, two physicians, two apothecaries, three female friends of the family, unmarried, advanced in life; and often in the nursery one clergyman, six flatterers, and a grandpapa. Less than this would not be decent." During the growing time of youth, the preponderance of nitrogenous matters present in milk makes it a most useful food; but in adult life this is not desirable. Yet, among Modem Methuselahs (Bailey), more than one centenarian is told about, who by eating but little substantial food, and only drinking milk, reached the great age of one hundred and thirty-eight years, whilst no hearty meat-eater has got beyond one hundred and three years.

The Casein, Or Curd Of These Milks

The Casein, Or Curd Of These Milks, is an albumin, but distinguished from other albumins by becoming coagulated when swallowed, through the action on it of the gastric juice, but not by heat when cooked, as the albumin (or white) of egg does. And the casein of one animal differs from that of another. This is the chief proteid, or nitrogenous constituent of milk, not coagulating spontaneously, as the fibrin, or albumin, of blood does, but by the action of acids, and of rennet. The casein of milk yields no uric acid, and does not contain any nuclein, which fact renders it specially of service for goutily disposed persons as a food; and it yields no carbohydrate when split up, which may render it of particular value in those severe cases of diabetes in which sugar is formed out of proteid foods. Curdling of milk depends on the production of lactic acid in the milk, which turns the curd, or casein, out of its partnership with the lime salts; then the casein, not being soluble, falls down as a flocculent precipitate, or clot. Lactic acid is formed in sour milk, this being, when concentrated by the chemist, a syrupy, intensely sour liquid, comprising well-defined salts. (It is produced likewise in the fermentation of several vegetable juices, and during the putrescence of some animal matters). Nevertheless, milk is the most powerful preventive of acidity, or neutralizer of acid, among all foods, chiefly by its citrate of lime, the basis of which is identical with that of lemon-juice; for a good cow yields practically as much citric acid in a day, as would be contained in two or three lemons.

This citrate of lime, as occurring in new unboiled milk, is altogether devoid of any sour taste.

The solid particles sometimes met with in Condensed Milk consist chiefly of this citrate of lime. The great majority of condensed milks are sweetened by the addition of cane sugar (indigestible by an infant) in considerable quantity, so as to preserve them unchanged after the cans have been opened. Children fed on condensed milk get their teeth late, and are likely to be rickety; they become plump, but paddy; large, but not strong, lacking the power of endurance, and of resistance to disease. The condensed milk, when used for emergencies, or in travelling, should always be diluted with at least ten times its bulk of water. In rickets, any excess of milk is to be avoided in the child's food, or aught else of the animal sort which causes stinking stools. The added cane-sugar prevents condensed milk from approaching the standard of human milk.

When milk enters the stomach, it sets into a solid clot, owing to the action on it, not at first of gastric juice, but of rennin; whilst the alkaline salts of the milk serve for a short time to partly neutralize the strong acid of the gastric juice, thus giving the rennin time to act, just as in making a junket with calf's rennet by the cook. Boiled milk clots more slowly outside the body than raw milk does, and with a less dense clot; but this is not the case within the stomach, as is often supposed by mistake, because the gastric juice redissolves the lime salts of the boiled milk, which then clots quite as firmly as does unboiled milk within the stomach. The change which takes place when milk turns sour by keeping, or in thundery weather, is caused by the growth in it of micro-organisms, which can be killed by heat (short of boiling). These micro-organisms are most active in milk at blood-heat, but scarcely at all in milk at 60° Fahr., and quite inactive at the freezing point. After being boiled, milk is free for a time from micro-organisms, but it will not remain so unless straightway sealed hermetically from the air, so as to prevent the entrance of fresh germs, which would shortly become very active therein.