THE ORGANIC CONSTITUENTS OF MILK 33 



colostral period, the proteins must be broken down, and the products 

 thus obtained be absorbed by the infant. It is difficult to afford 

 a direct proof of this statement, but indirect and convincing proof 

 is afforded by the fact that the mechanism which is concerned 

 with the production of immunity appears to be intimately attached 

 to the protein constituents of the blood. It will be shown more 

 fully in another chapter that these properties are present in the 

 milk, presumably attached to the proteins, and that during the 

 early days they can be transferred to the infant and can be found 

 in the infant's circulation. Had the proteins been subjected to 

 digestion along ordinary lines, the properties connected with 

 immunity would almost certainly have been destroyed. Hence 

 the assumption that these substances have been absorbed directly 

 into the infant's body. This absorption does not, however, take 

 place after the first few days of life. 



2. On the Pats of Milk. The percentage of fat in milk has 

 been considered in the preceding chapter. The composition of 

 fat in the milk of different species varies considerably, being com- 

 posed of different fatty acids. It would not appear necessary to 

 enter upon these differences in composition, since no evidence has 

 been adduced which renders it of importance from the nutritional 

 standpoint, and the investigations upon the subject are not numerous. 

 The amount of fat given has been shown to vary considerably in 

 the twenty-four hours. 



3. Sugar. The carbohydrate found in milk is lactose. This 

 substance is present in greater amount in human milk than in 

 cows' milk. 



CHAPTER III 



THE ORGANIC CONSTITUENTS OF MILK 



IN the preceding chapter the different classes of chemical sub- 

 stances present in milk have been considered as a whole. This 

 general aspect is, however, not sufficiently precise for the present 

 purpose, and it will be necessary to devote attention to the in- 

 dividual organic and inorganic substances comprised in the general 

 classes. The organic substances fall into the three main classes 

 of food-stuffs proteins, carbohydrates, and fats and will be dealt 

 with in the present chapter under these headings, the inorganic 

 constituents being considered in the following chapter. 



i. The Proteins and Nitrogen-containing Substances. The 

 discovery of methods for the separation of the proteins of blood 

 and of egg-white were applied to milk by various observers. An 

 account of the development of this line of research is given by 



