The study of the bacteriology of the alimentary canal in the infant and child is so essentially a study of the behavior of bacteria in varying conditions that a consideration of the normal and abnormal bacteriology of milk is of the first importance. Since milk is the natural diet of infancy, we are compelled, in endeavoring to appraise the actual nature and extent of the pathological condition found in any given case, to consider the precise circumstances in which the same or allied processes occur in milk outside the body; for, by experimentally reproducing in the laboratory test-tube the biochemical processes taking place in the alimentary canal, we are seeking to establish these fundamental correlations which are essential to accurate and scientific diagnosis. It is further of importance that we should be able to appreciate the nature of the biochemical processes occurring under normal conditions, for we can scarcely be