The food of infants comes in the main from two sources, human milk and cows' milk, the former being the natural food and the latter the best substitute we have when the former supply fails us. A brief description of these two milks is necessary for a clear conception of the subject. The percentage of fat in cows' milk varies with the individual cow and the species. The milk of the ordinary grade cow, the Holstein or Ayrshire, contains 4 per cent, or less of fat, and that of the Jersey or

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... y 4 per cent., or more, of fat. The fat is held in a coarser emulsion and separates more easily than that of human milk. There are other differences in the chemical and physical composition of the two milk-fats, but these will not be taken up in this paper. Human milk normally contains, in round numbers, about 4 per cent. of fat; this percentage, however, may vary considerably in pathologic conditions, the extremes being 0.1 per cent.1 and 13.7 per cent.2 Every woman has a certain amount of fat to secrete in her milk, and, under like conditions, gives the same total day by day. The percentage of fat in the milk gradually increases in a certain definite curve from the beginning to the end of each nursing, the smallest percentages coming at the begin¬ ning and the highest at the end.2 Por example, the first part of the milk usually contains 2 per cent, of fat, and the last part 6 per cent, of fat; and the mixture of the whole amount of milk secreted in a single breast contains 4 per cent, of fat. The amount of milk secreted does not influence this curve otherwise than by making the ascent rapid or