With respect to the vitamins, it is the general opinion that these pass into the milk only as supplied from the food of the mother, and that the mammary gland has no power of synthesizing these essentials.61

Since A is present to some extent in the body fats of animals it is probable that small amounts of this will be secreted in the milk, for a time at least, even if it is not supplied in the food, but the experience of McCollum and Simmonds above referred to, indicates that under these circumstances it is not sufficiently abundant for the optimum growth of the young. Hopkins 62 found that the vitamin A of goat's milk remained approximately constant when the animal was given a diet deficient in that factor, although the quantity of milk fell off, but the length of the experimental period is not stated.

If we accept the hypothesis that rickets is connected with deficiency of A, then the observation of Hess 63 as to the prevalence of rickets among the breast-fed children of the negro women whose diet is limited almost exclusively to substances poor in A, must be regarded as significant.

Dutchcr, Kennedy and Eckles64 have found that spring milk, produced when the cows are pasturing on fresh grass, is superior to winter milk from cows on dry feed with regard to both A and B. The tests were made by comparing the growth of rats on a ration of casein, dextrin, agar agar, butter-fat, extract of wheat embryo, and an adequate salt mixture, with that of other groups in whose diets either the butter-fat or the wheat embryo had been replaced by varying quantities of winter milk or spring milk. When the spring milk was at its best 10 cc. per day furnished sufficient of both A and B for normal growth, but butter-fat from winter milk had to be fed to the extent of 20 per cent of the ration in order to secure growth, and even this amount was not always adequate.65

Osborne and Mendel66 were unable to observe any difference between summer and winter milk in the relative amounts of B present, which was perhaps to be expected from the relative stability of this vitamin and its wide distribution in the grains which make up a part of the ordinary winter feed of dairy cows. Dutchcr, Eckles, Dahlc, Mead, and Schaefer67 state, however, that in their experience the content of B in the milk of cows is influenced by the character of the ration, although less markedly than is true of the vitamins A and C, and Andrews 68 has shown that nursing mothers on a diet deficient in antineuritic B produce milk on which their nurslings develop beriberi.

61 See McCollum, Simmonds and Pits, J. Biol. Chem. 27, 33, 1916. 62 Quoted by Harden, J. Soc. Chem. Ind. 40, TOR, 1021. 63 Heat, J. Am. Med. Assoc. 70, 900, 1918.

64 Dutchcr, Kennedy, and Eckles, Proc. Am. Chem. Soc. Sci. 52, 588, 1920. 65 See also Hess and linger, J. Am. Med. Ass. 74, 2171, 1920. 66 Osborne and Mendel, J. Biol. Chem. 41, 515, 1920. 67 Dutcher, Ecklcs, Dahlc, Mead, and Schaefer, J. Biol. Chem. 45, 121, 1920.

Several investigators have reported a seasonal variation in the antiscorbutic properties of cow's milk,69 and this point may be regarded as established. Hart, Steenbock and Ellis found that while 50 cc. per day (and in some cases less) of summer pasture milk afforded protection to guinea pigs against scurvy, at least 75 cc. of winter feed milk were required to provide the same degree of protection. They remark the fact that the milk produced from such dry foods as are used does contain a measure of antiscorbutic vitamins shows that the dry feeds themselves are not completely devoid of this factor. "Their content in this vitamin is too low for protection of a guinea pig against scurvy, but the mammary gland, through a process of special selection and concentration in the milk produced becomes an effective means of showing the presence of C, especially in old dried feeds."Neither silage nor sugar mangels were notably effective in increasing the proportion of antiscorbutic in the milk. Barnes and Hume™ suggest that the use of turnips rather than mangels should result in the production of milk of higher antiscorbutic value.

Dutcher and his collaborators found that there is a decided tendency for the nutritive properties of the milk to remain relatively good for four to eight weeks after the cow has been placed on a vitamin-poor winter ration. On the other hand, the nutritive superiority of spring milk became evident almost immediately after the cows were placed upon green grass.

The question of the effect of heat on the nutritive value of milk is of the greatest practical importance in an age when the use of pasteurised, condensed, evaporated, and dried milks is constantly increasing, and raw milk of good quality is increasingly difficult to secure.

The pasteurisation of milk is usually carried out either by heating the milk rapidly to a point not far short of boiling, maintained there for a few seconds and then cooled as rapidly as possible (the " flash " method of pasteurisation), or by heating to 60° to 63° C. (140° to 145° F.) and holding at this temperature for 20 to 30 minutes or even longer (the "hold" method). The temperatures used may vary considerably from those mentioned but the essential difference between the two methods lies in the fact that in the one the milk is heated rapidly to a high temperature and cooled immediately, while in the other it is held at a lower temperature for a much longer period. The "hold " method is usually recommended as preferable on account of the lower temperature involved, but the work of Daniels and Lough-lin referred to below (p. 282) indicates that this is a mistaken point of view. The "hold" process is the one most generally employed commercially in the United States, while the "flash " process is more generally used in Great Britain.

68 Andrews, Phil. J. Sci. 7B, 67, 1912.

69 Barnes and Hume, Lane. 1919, ii, 323; Dutcher, Eckles, Dahle, Mead, and Schaefer, J. Biol. Chexn. 46, 119, 1920; Hart, Steenbock and Ellis, Ib 42, 383, 1920; Hess, linger and Supplee, lb., 46, 229, 1920.

70 Barnes and Home, Bioch. J. 13, 306, 1919.