Mother's milk is the food best adapted to the needs of infants and there is no perfect substitute for it. Not every mother, however, has enough milk for her child, or milk of a quality that adapts it to the child's needs. It is, therefore, necessary to study somewhat in detail the subject of human lactation and the effect upon infants of variation in the composition of milk, before taking up the subject of substitutes for human milk.

From 700 to 2000 c.c. (27 to 65 ounces) of milk are secreted daily by a mother's breasts. The latter is an unusually large amount. The average child requires during the first weeks of life from 250 to 500 c.c. (8 to 16 ounces) of milk, and in the tenth, eleventh, and twelfth months from 1000 to 1500 c.c. (30 to 50 ounces).

The milk secreted by a mother's breast during the first three days after childbirth is called colostrum. It differs radically from that which is formed later. It is slightly laxative. The composition of colostrum has not been definitely determined, but a sufficient number of chemical analyses have been made, however, to show that it varies much. It is usually poor in fat. This causes it to look bluer than other milk. Under the microscope it appears, like milk, to be a fine emulsion of fat, but it also contains large cells called colostrum cells. They gradually disappear during the second week of lactation and rarely reappear. If they do, they indicate a disturbance of lactation, and the milk containing them is especially likely to disagree with the suckling.

Mother's milk, like cow's milk, varies somewhat in composition during the act of nursing. The 'fore-milk,' or that which is obtained at the beginning of the process of milking, is richest in fat and contains less water. The variation in fat is not due altogether to the greater dilution of the 'fore-milk,' for the increase in fat in 'strippings' over 'fore-milk' may be more than 100 per cent., while the difference in fluidity is relatively slight. During the first week of lactation, milk is usually richer in proteins and poorer in fat and sugar than it is later. During the next week or two, the sugar reaches its normal percentage. Fat gradually increases and reaches its equilibrium during the second month. The percentage of protein falls simultaneously with the increase of fat.

Analyses made for Dr. Effa V. Davis1 of milk furnished from eight mothers on the fourth day of the puerperium averaged:

The protein varied between 1.21 per cent, and 2.67; fat between .20 and 2.40; and sugar between 3.58 and 8.25. Analyses made of the milk from five women drawn on the fourteenth day averaged:

The protein varied between 1.47 and 2.65; the fat between .20 and 4.00; and the sugar between 2.74 and 8.86 per cent. Analyses made for Holt gave the following averages:

So long as lactation is normal the milk will vary little until the last months of nursing, when the percentage of proteins and fats decreases, although the total quantity of milk secreted increases.