GENERAL METABOLISM. 657 



some time. In spite of this different behavior of the casein from the two 

 kinds of milk, which may be due to several causes, we are not justified in 

 assuming that there is any great difference in the nature of the casein. 



On the other hand, just as we cannot safely assume from our present 

 chemical knowledge that the composition and nature of the protein from 

 different kinds of milk are dissimilar, so we are not justified in assuming 

 that the milk of the different species of animals is quantitatively but not 

 qualitatively different. The present state of our knowledge concerning 

 the composition and nature of the different constituents of milk does not 

 tell us how completely the milk of one species may be replaced by another. 

 This does not by any means imply that it is impossible to effect a satis- 

 factory replacement. We only wish to emphasize at this place how far 

 our present knowledge is from the desired goal, and how far the present 

 demands and concessions have stretched beyond the boundaries of our 

 actual knowledge. At present we are obliged to depend almost entirely 

 upon practical experience which receives but slight support in the ana- 

 lytical values obtained from the investigation of milk. We must emphasize 

 the faot that our present methods of examining milk, particularly the 

 analysis of the ash, show us merely what elements are present and in 

 what proportions. The presence of sulphuric acid in the ash may be 

 accounted for in several ways. It may occur in the milk as such, or the 

 sulphur may be present in some state of combination other than that of 

 sulphate. ' On the other hand, the old idea that the intestine is only able 

 to bring about certain slight changes in the food, which is then absorbed 

 after having been reconstructed as little as possible, is more and more to 

 be discarded. As a matter of fact, the changes which take place while 

 the food is in the intestine are quite considerable. The assimilation begins 

 in the intestinal canal. The synthetic capabilities of the animal organism 

 are much greater than was formerly assumed. It is far less dependent 

 upon the nature of the food which it receives than was once believed to 

 be the case. 



Practical experience has shown that it is not possible to replace entirely 

 satisfactorily the mother's milk with that of some other species, or by means 

 of a milk substitute. The mortality of infants nourished at the breast is 

 much less than that of infants brought up in some other way. It is an 

 open question, however, whether this increased mortality is wholly due 

 to insufficient nourishment. In many cases it is perfectly true that the 

 children of women who are not able to nurse their children, or at least only 

 for a short time, are in many cases not as strong as the children of normal 

 women. Statistics in this direction, therefore, to be useful must take into 

 account not merely whether the child was brought up on mother's milk, 

 or upon a milk substitute, but it should also be stated why the mother's 

 milk was abandoned. It is perfectly clear that if a sickly child is placed 



