It may be stated as a generally accepted axiom that the more closely the artificial diet of an infant approaches in composition that of breast milk, the more likely is it to be a successful substitute. Certain modifications are called for in connexion with cows' milk in order to secure this object. We shall now deal with these, taking first those which are simple and easily carried out at home, and next those which are more elaborate and require skilled supervision. It will be necessary to consider the effect of these modifications, and whether the avoidance of certain dangers may not be accompanied by a tendency to others. For example, the sterilization of cows' milk in order to prevent the danger of tuberculous infection may remove entirely from the milk the quality of freshness or vitality in which lies its antiscorbutic property. The questions raised are not purely chemical ones and cannot be determined by laboratory experiments; they are also biological and can only be settled by testing them on many infants over prolonged periods. At present there is no consensus of opinion as to the best modification of cows' milk, and each writer on the subject must be guided to a large extent by his own experience. He should also pay due attention to the experiences of others with different methods, and should write without dogmatism.
The Essential Difficulty About Cows' Milk lies in the quantity and quality of its protein matter. Dilution with fluid is the simplest manner of reducing the quantity to the standard of breast milk, while as a matter of clinical experience the quality of the proteins is not found to require modification in the case of healthy children. It is different with delicate or ailing infants, who will be considered later. For the first fortnight of life the dilution may be of the strength of milk : diluent as 1:3, and then on to the age of three months 1 : 2. From the age of three to six months, equal parts may be given, and from six to nine months two parts of milk to one of diluent. The diluents in common use are plain water, barley-water, and lime-water.
Plain water may be used, either boiled or unboiled, according to the purity of the water supply, but as a rule custom dictates some other diluent. Barley-water has come into very common use. It is supposed to diminish the density of the milk clot formed in the stomach and thereby to make digestion more easy. The nutritive value popularly attached to it by mothers is mythical, for if properly prepared, barley-water will contain less than 1 per cent of starch. It should be prepared as follows : A tablespoonful of pearl barley is washed and put in a saucepan with one pint of cold water. This is brought to the boil and then allowed to simmer beside the fire for half an hour. The water is then strained off and used as required. Barley-water should be prepared twice daily as it does not keep well. Strong decoctions of barley, or those made from ground barley, are not to be recommended for infants until after the age of nine months. Before that they are the cause of much intestinal disturbance as the digestive apparatus for starch is not sufficiently developed. The chief benefits in using barley-water are that it is a bland fluid, less irritating to the stomach than tap water, and that as it has been boiled the fluid is sterile. Lime-water is also less irritating in the stomach than plain water. The lime has probably a slight effect in delaying the curdling by the acids in the stomach, but its antacid power is very feeble. It may be used quite safely as the regular diluent, but as a rule is better reserved for cases with gastric disturbance.
The Dilution Of The Milk renders the addition of some fatty matter necessary, as the fat has been reduced to one-third the amount in breast milk by the addition of two parts water. It is often advisable, however, to begin bottle feeding in the first week without the addition of any cream, as the fatty material so often causes digestive disturbances. One can thus ascertain first of all whether the protein contents of the artificial food are going to be well tolerated by the infant, and can add the fatty material in gradually increasing amount as toleration is established. Digestive disturbances are but too often started by the tendency to feed up an infant from birth, which usually means overfeeding, and the tendency to give an excessive amount of fat at this age is specially marked. The extra fat is usually added in the form of cream, butter, or cod-liver oil. A pint of good cows' milk will yield from 1 1/2 to 2 oz. of gravity cream. Gravity cream contains from 15 to 20 per cent of fat and the same amount of proteins and sugar as the milk. If 7 oz. of milk diluted with twice that amount of water are used, 1 oz. of cream must be added to the mixture to bring the milk to the proper standard, i.e. 4 per cent of fat. The exact strength of the cream in fat is an important matter and Holt has found by careful experiments that he can get a standard 20 per cent cream as follows. If cows' milk from a mixed herd is put into a pint vessel soon after it is drawn and rapidly cooled, it will be found that after four hours the upper 4 oz. will contain 20 per cent of fat. Unless, however, one is dealing with a milk of known strength it is impossible to estimate the amount of fat in gravity cream with absolute accuracy. Still it serves well enough in ordinary circumstances and gravity cream has the advantages of being digestible and easily made at home in four hours from fresh milk. Separated cream can be made of any definite strength in fat, although as commonly sold the fatty contents may vary from 40 to 60 per cent. It has the disadvantages of being less easily procured in many places, of being often stale from keeping, and of being less digestible than gravity cream from a change in the fat globules through the action of the centrifuging machine.
Cream is a very expensive luxury for the poor and cannot always be obtained of good quality even by the well-to-do. Butter or margarine may be used in place of it to supply the extra fat. Butter and margarine contain about 82 per cent of fat, i.e. twice as much as separated cream and four times as much as gravity cream. In infant feeding, if butter or margarine is used, one must add one quarter the amount as compared with gravity cream and one half the amount as compared with separated cream. Both butter and margarine are easily digested and possess great nutritive value. If good butter is sometimes rather expensive, the price of margarine places it within the reach of all. Cod-liver oil is a cheap and efficient substitute for cream, and is easily digested by most infants.