The more elaborate methods of modifying milk require careful consideration because they represent attempts to overcome difficulties in connexion with this food material. Some of them are of a scientific nature and are based on chemical and bacteriological investigations. Others are of a commercial nature with a pseudo-scientific claim attached to them by the manufacturers. There are two standards by which these preparations must be judged, first as to how closely they approximate in composition to breast milk, and secondly as to their effects, immediate and remote, on the growth and nutrition of the infant. It will be found that in correcting one fault or defect in cows' milk, the result too often is that others are produced. In other words, clinical experience has shown that scientific modifications of milk are not to be relied on as the sole guides in infant feeding. Test tube experiments in the laboratory are not the same as digestive experiments in the human stomach. Chemistry and bacteriology are not enough without considering the biological needs of an infant, and these can only be determined by prolonged observations on a living child.

We have already considered certain modifications of milk which are simple and can be carried out in any household. They have stood the test of experience and may be regarded as leading to the production of a diet which will carry the infant safely through the first nine months of life, and will not tend to ill-health or disease afterwards. There are certain other modifications, more elaborate as a rule, which have been evolved with the object of meeting certain difficulties in connexion with feeding with cows' milk. These will now be considered in detail, attend tion being directed to the method of preparation, the advantages which are claimed, and the disadvantages which may accompany the use of milk thus modified. We shall deal here only with the use of these modifications of milk in the feeding of healthy children, and shall defer the question of their application to the treatment of diseased conditions to a later section.