There is no perfect substitute for human milk. On page 69 the composition of the milk of several animals is compared with that of woman. None approaches the latter closely. Cow's milk is the best substitute for human milk, because it is abundant, cheap, and easy to procure. After the first three or four months many infants can digest it comfortably and thrive upon it. Others cannot take it unmodified until they are a year or more old.

It is necessary to study with care the difference in composition and character of cow's milk and of human milk. They differ chemically, as the following analysis shows:

Cow's milk contains much more protein and mineral matter than does human milk, a little more fat, and less sugar. It is acid, not alkaline, in reaction. Cow's milk and human milk differ also in the character of the proteins. It has been estimated that one-eleventh of what is commonly estimated as protein in human milk is extractive matter, the exact nature of which is unknown. The proportion of casein to albumin in human milk is approximately as 1 is to 2. In cow's milk casein is to albumin as 6 is to i. The casein of cow's milk makes, with acid, large masses of cheese, which are dissolved with difficulty by an excess of acid; that of human milk forms a fine, loose, small, flocculent mass that is readily soluble in an excess of acid. The denser larger masses of casein obtained from cow's milk are due to the larger proportion of casein in it, and to the relatively smaller percentage of fat and soluble albumin, which acts mechanically to make a looser clot, and to the fact that cow's milk contains six times as much lime and three times as much acid as human milk. The fat of human milk contains more oleic acid, is in a finer state of emulsion, and is more digestible than that of cow's milk. The mineral matter of the two kinds of milk is different, but not to such an extent that this difference becomes of great importance.

The density and size of the curds of casein in cow's milk are lessened by diluting it before acid is added. If cow's milk is diluted with five parts of water, acetic acid will produce no curd perceptible to the unaided eye. The reaction is similar in all respects to that obtained by adding the acid to undiluted human milk. If smaller proportions of water are added to cow's milk, perceptible curds will form when acetic acid is mixed with it, but these grow smaller and looser in proportion to the quantity of water added to the milk. Such dilution makes the protein much more digestible, but it also lessens the percentage of all nutritive ingredients in the milk mixture. For instance, cow's milk that contains on the average 4 per cent, of fat, 4.5 of sugar, and 4 of protein will, if diluted with five parts of water, contain less than 1 per cent, of each of these ingredients. Even equal parts of cow's milk and water will lower the percentage of fat and sugar below that of average human milk, and leave in the mixture twice as much protein as is desirable.

But the experience of pediatricians has shown that the proteins of cow's milk are not as universally indigestible as was formerly supposed. Indeed they are often well digested by feeble infants who on the other hand cannot digest fats.

The average infant, however, needs milk modified by dilution, by the addition of milk sugar and fat in the form of cream. Many years ago John Forsyth Meigs, of Philadelphia, found that milk diluted with water and strengthened with cream and sugar agreed more frequently with delicate infants than anything else. An analysis of the mixture thus empirically used showed that he had hit upon a combination of fat, sugar, and protein that resembled closely that of human milk. His son, Arthur V. Meigs, published a paper calling attention to these facts and to the errors in analyses of milk then currently accepted, and laid the foundation of the scientific modification of milk for infants' use. Twenty years ago Rotch urged that physicians think of all milk mixtures in percentages of their proximate principles. This is of the greatest utility, as it leads one to compare constantly the given mixture with average human milk, and when any ingredient is increased or lessened in amount, it is so changed with reason - that is, to alter the percentage of fat, or of protein, or of sugar. Nothing has contributed more to save infants' lives during recent years than furnishing them with clean, pure cow's milk, and its percentage modification. Various formulas have been devised by means of which percentage modification can readily be made by mixing cream, skimmed milk, milk-sugar, and water. In many of the larger cities of this country milk laboratories have been established, where milk, modified according to any prescription, can be procured, and where the modification is made by chemists, with as much accuracy as a prescription for medicine would be compounded by a pharmacist. In these laboratories milk is sold that is obtained from herds of healthy cows. Both animals and milk are handled with the greatest care, to insure the cleanliness and purity of the latter. By a centrifuge, cream is separated from the milk, which contains uniformly a given percentage of fat. With this cream, milk, mik-sugar, lime-water, and distilled water the prescriptions for milk mixtures are filled. Some physicians, however, prefer 'gravity cream,' and so specify in prescriptions.

The prescriptions should specify the percentage of fat, sugar, protein, mineral matter, and degree of alkalinity desired in the mixture. At the laboratories the milk will be Pasteurized if it is desired. As soon as a given milk mixture is prepared, so much is poured into a nursing-bottle as is needed for one feeding, and as many bottles are filled to this extent as the infant for whom it is intended will need during twenty-four hours. The flasks are plugged tightly with nonabsorbent cotton and put into a refrigerator. As many bottles are delivered daily at the home of the infant as are needed. When one is required for use, it is taken from the ice-chest, warmed sufficiently, the cotton plug is removed, and a rubber nipple fitted to it. In this way a minimum handling of the milk is insured, and when it is handled and changed from receptacle to receptacle, it is done by those who are trained to the work, and where every possible precaution is taken to insure cleanliness. All receptacles used for milk at the farm and in the laboratory, and the nursing-bottles as well, are perfectly sterilized in large ovens before they are used. The results that are obtained from the use of milk thus modified and thus carefully guarded against contamination, are naturally much better and more uniform than are gotten by older methods, or by methods that fail to insure equally rigorous supervision.