484 FOOD [CH. XXX. 



kingdom. The quantitative variations are often enormous, and it 

 has been shown that the milk best adapted for the nutrition of the 

 young animal is that which comes from its mother, or, at least, from 

 an animal of the same species. The practical application of this 

 rule comes home most to us when dealing with the feeding of children, 

 and it is universally acknowledged that, after all, cows' milk is but 

 a poor substitute for human milk. Cows' milk is, of course, diluted, 

 and sugar and cream added, so as to make it quantitatively like 

 mothers' milk, but even then the question arises whether the 

 essential difference between the two kinds of milk is not deeper than 

 one of mere quantity; and, in particular, the pendulum of scientific 

 opinion has swung backwards and forwards in relation to the 

 question whether the principal protein, called caseinogen, in both is 

 really identical in the two cases. The caseinogen of human milk 

 curdles in small flocculi in the stomach, so contrasting with the 

 heavy curd which cows' milk forms; and even although the curdling 

 of cows' milk be made to occur in smaller fragments by mixing the 

 milk with barley water or lime water, its digestion proceeds with 

 comparative slowness in the child's alimentary canal. These are 

 practical points well known to every clinical observer, and in the 

 past they have been attributed, not so much to fundamental 

 differences in the caseinogen itself, as to accidental concomitant 

 factors ; the excess of citric acid in human milk, for instance, and its 

 paucity in calcium salts, have been held responsible for the 

 differences observed in the physical condition of the curd and in its 

 digestibility. 



This question is far from settled even to-day, but there are some 

 data now available that point to a qualitative difference between 

 caseinogens. Some of these depend on the application of the 

 " biological test " carried out on the line of immunity experiments, 

 which has been so signally successful in the distinction between the 

 blood-proteins of different species of animals (see p. 477). The 

 differences, however, which lead to the formation of specific pre- 

 cipitins are so slight, that ordinary chemical methods of analysis are, 

 at present, unable to reveal them. But, in the case of milk, there 

 are differences which the chemist can detect. One cannot lay much 

 stress on mere percentage composition, although differences have 

 been noted in that, because we have no guarantee that the proteins 

 investigated were separated from all impurities ; there are also small 

 differences in the percentage of mono-amino-acids obtained after 

 hydrolysis ; but the present methods of estimating these with 

 accuracy leave much to be desired. A deeper chemical distinction 

 noted is contained in the recent work of Bienenfeld, who finds that 

 human caseinogen contains a carbohydrate complex which is absent 

 from that of the cow. 





