With the exception of iron,32 the various salts appear to be present in greater amount in cow's milk than in human milk. In particular the important elements, calcium and phosphorus, are both greatly in excess in cow's milk.

32 Spxhlet, Munch. Med. Wochensch. 28, 1529; 1912.

All three vitamins, A, B, and C, are present in milk. A, as noted above, is associated with the butter-fat, and presumably varies in amount with varying fat content.

There is a surprising divergence of opinion among different authorities as to the proportion of B present. Hopkins 33 found that 2 cc. of milk per day, or an amount furnishing the equivalent of from one to three or four per cent of the total solids in the ration induced satisfactory growth in rats on a diet of casein, fat, starch, sugar, and inorganic salts. Without the milk there was no growth on this diet. Somewhat comparable results were obtained by McCollum and Davis 34 who report that as little as two per cent of skimmed milk powder in a ration of polished rice, casein, butter-fat, and salts produced nearly normal growth. On the other hand, Osborne and Mendel35 found that a diet of casein, starch, butter-fat, lard, and salt mixture had to be supplemented with as much as 24 per cent of dried whole milk or 16 cc. per day of fresh milk in order to induce rapid growth in rats. That vitamin B is the decisive factor is indicated by the success attained when the milk is replaced by small doses of brewers' yeast: 02 gm. of dried yeast proved more effective in promoting growth than 15 cc. of milk.

The work of Hopkins and McCollum and Davis, together with the early observations of Osborne and Mendel on the growth-promoting power of " protein-free milk," gave rise to a wide-spread belief that milk was peculiarly valuable as a source of B. The later work of Osborne and Mendel just referred to indicates, however, that this is at any rate not invariably true, and that it is wise to assure safety either by the very liberal use of milk or by the addition to the diet of some other source of B.

When tested for its antineuritic power milk shows little efficiency •• which would appear to confirm the results of Osborne and Mendel, unless we assume the growth-promoting and antineuritic power to be due to two distinct factors.

All evidence goes to show that milk is poor in C, although by no means devoid of this vitamin. One hundred to one hundred and fifty cc. per day were required to protect guinea pigs from scurvy, and a little more than this was usually required to afford protection to monkeys." Experience in human nutrition has shown its great inferiority to fresh fruit and vegetable juices in this respect,38 although it is effective if given in sufficient amount.39

33 Hopkins, J. Physiol. 44, 426, 1912; Hopkins and Neville, Bioch. J. 7, 97, 1913. Being subjected to criticism by Osborne and Mendel, who claimed that more than 2 cc. of milk is necessary, Hopkins has confirmed his previous work. (Biochem. J. 24, 721, 1921).

34 McCollum and Davis, J. Biol. Ghem. 23,181, 1915.

35 Osborne and Mendel, J. Biol. Ghem. 34, 637, 1918; 41, 516, 1920.

36 Cooper, J. Hyg. 12, 436, 1912; 14, 12, 1914; Gibson and Concepcion, Phil. J. 8c. 11B, 119,1916; Chick and Hume, J. Roy. Med. Corps, 29, 121, 1917.

Mattill and Conklin40 found that while cow's milk appeared to be entirely satisfactory for the growth of young rats from weaning (when about 26 days old) to about the fiftieth day of life (equivalent to about five years of human life) a decided retardation of growth and sometimes a considerable loss of weight occurred later, usually between the fiftieth and one hundredth days. After this growth might be resumed again, but at a slower rate, and there was no reproduction. Milk powder was somewhat more satisfactory than fluid milk, males growing at a practically normal rate on this, though females were somewhat below normal after about 75 days. This is in accord with the observations of Lane-Claypon,41 who found that rats made slightly better progress on dried milk than on fluid milk. Mattill and Conklin found that reproduction failed on dried milk as on fluid milk. The average daily intake of milk, computed in terms of milk solids, was somewhat less when this was given as fluid than when fed as powder, which may doubtless account for the better progress on the latter food. Addition to the milk of certain inorganic salts was without effect, except in one or two cases where additional iron proved beneficial for growth, but had no effect on reproduction.

Addition of yeast or wheat embryo to supply additional B increased the rate of growth slightly, but the improvement was only temporary. One female became pregnant on the yeast-milk diet, but the young were eaten. Addition of as much as 10 per cent of butter-fat to the ration of dried whole milk failed to induce continued normal growth, but a ration of 63 per cent dried milk, 6 per cent butter-fat, and 40 per cent starch proved entirely satisfactory for growth. Reproduction took place, but the young were eaten by the mothers. Osborne and Mendel42 have, however, reported successful growth, continued in the second generation, on a somewhat similar diet of 60 per cent milk powder, 12 per cent starch, and 28 per cent lard. It would seem therefore that milk lacks something which is essential for growth during part, at least, of the rat's life and for fertility. It is improbable that this factor is identical with the recognised vitamins, since it is apparently supplied by starch. When the starch is replaced by lactose to the extent of 25 per cent, the remaining 75 per cent of the food being obtained from dried milk, all growth stopped.

37 Barnes and Hume, Bioch. J. 13, 906, 1919.

38 Carran, Dub. J. Med. Sci. 7, 83,1847; Barlow, Lane. 1894, ii, 1075.

39 Comrie, Ed. Med. J. 24, 207,1920.

40 Mattill and Conklin, J. Biol. Chem. 44, 7, 1920.

41 Lane-Claypon, J. Hyg. 9, 238, 1909.

42 Osborne and Mendel, J. Biol. Chem. 15, 811, 1913.