Hart and Humphrey3 have shown that the protein content of the milk varies very little even though a cow may be losing her own flesh to furnish the milk. Thus, when a cow was given a food with a "nutritive ratio" of 1 :8, that is, 1 part of protein to 8 parts of carbohydrate and fat, a positive nitrogen balance was present provided milk protein was given in the diet, but when protein was administered in the biologically lower form of wheat protein, a negative nitrogen balance resulted. The quantity of protein in the milk, however, remained unchanged.

These facts are shown in the following table:

"Milking the flesh off the back" is, therefore, a reality. During lactation a ration high in protein is wisely dictated and the biologic status of the protein must also be considered.

According to similar laws, Fingerling4 finds that a fodder deficient in calcium has no effect upon the calcium content of the milk, the organism providing this material. Furthermore, Lauder and Fagan1 found that the addition of 225 grams of calcium phosphate to a fodder already containing the same content of that salt did not alter the calcium content of cows' milk. Von Wendt2 states that ingestion with the fodder of sodium chlorid, calcium carbonate, calcium hydrogen phosphate, calcium glycerophosphate, sodium phosphate, or magnesium bromid is without definite influence upon the composition of the milk. The lactic glands, therefore, prepare a fluid of very definite composition specifically designed for the offspring of the species.

1 Gogitidse: "Zeitschrift fur Biologie," 1904, xlv, 365.

2 Gogitidse: Ibid., 1905, xlvi, 403 .

3 Hart and Humphrey: "Journal of Biological Chemistry," 1915, xxi, 239.

4 Fingerling: "Landwirtschaft. Versuchsstationen," 1911, lxxvi, 1.

How may the various effects of diet be explained? The subject requires a knowledge of the processes going on in the mammary gland, and these are not certainly known. It has been generally believed that the cells of the mammary glands undergo a fatty metamorphosis and, themselves breaking up, pass into the milk (Voit, Heidenhain). The milk under these circumstances might be regarded as the substance of an organ, made fluid.

Schafer,3 however, believes the process to be one of secretion similar to that in the salivary glands, where the cells prepare the special constituents and pass them on to the lumen. Thus casein, like ptyalin, may be specially elaborated within gland cells.

If this be the true explanation, the influence of food, in the writer's opinion, may be readily explained. An increased protein ingestion furnishes the digestive products of this substance in liberal quantities and may increase the activity of the gland.

The milk-sugar content of the milk remains remarkably constant. Cremer,4 for example, has shown that the percentage of milk-sugar in the milk is unchanged in the cow after diminishing the sugar content of the animal by inducing phlorhizin diabetes.

1Lauder and Fagan: "Proceedings of the Royal Society of Edinburgh," 1914-15. xxxv, 195.

2 von Wendt: "Skan. Archiv fur Physiologie," 1909, xxi, 89.

3 Schafer: "Text-book of Physiology," 1898, i, 667.

4 Cremer: "Zeitschrift fur Biologie," 1899, xxxvii, 78.

To explain the fat content of the milk the writer offers the following theory: When for any reason sufficient sugar is not oxidized in the body cells, these sugar-hungry cells attract fat. It has already been seen that the glycogen and fat content of the liver are antagonistic. Before lactation sets in, the cells of the mammary glands oxidize sugar and there is no great attraction for fat. It is believed that milk-sugar cannot be formed in any great quantity before parturition, because it occurs in the urine only postpartum.1 That milk-sugar is not formed outside of the mammary glands was demonstrated by Moore and Parker,2 who completely removed these glands from a goat during the period of gestation, and later at the time of parturition found no sugar in the urine. Had milk-sugar, which cannot be oxidized by the organism, been formed outside the glands it would have accumulated in the blood and have been eliminated in the urine. When in the process of lactation the glucose furnished by the blood is converted into milk-sugar (which cannot be burned within the organism), the mammary cell becomes a sugar-hungry cell which at once attracts fat from the blood. This theory of the writer explains the production of milk fat by the process of infiltration. The variation of the percentage of fat in the milk may be explained by the quantity of fat in the blood. During starvation the blood becomes rich in fat on account of the transportation of tissue fat to the cells. Administration of sugar at once reduces the supply of fat in the blood. But if fat be ingested with carbohydrates the blood becomes rich with this fat and affords material for a rich milk.

Administration of good cream with a substantial mixed diet is highly to be recommended for nursing mothers. The daily production of a liter of milk, which has a value of 640 calories, indicates the necessity of no small addition to the daily ration, if the woman is to bear satisfactorily the strain of lactation. Probably this extra nourishment is best given in the form of fat.

1 Lemaire: "Zeitschrift fur physiologische Chemie," 1896, xxi, 442.

2 Moore and Parker: "American Journal of Physiology," 1900, iv, 239.

Should the fat of the milk disagree with the infant, the trouble may be due to the kind of fat ingested by the mother. If, however, the indigestion be due to a large percentage of fat, a carbohydrate diet may be used to reduce the percentage in the milk.

It may be added that Voltz and Paechtner1 report that after moderate ingestion of alcohol only minimal quantities of it are found in human milk.

1 Voltz and Paechtner: "Biochemische Zeitschrift," 1913, lii, 73.