466 



NUTRITION OF FARM ANIMALS 



phates) may be utilized as sources of the phosphorus of the milk 

 (257, 258, 497). 



The production of casein, however, is not simply ,a conju- 

 gation of a simple protein with a phosphorus group. The 

 constitution of casein is markedly different from that of the 

 proteins of the blood serum or of the muscles, as is shown by 

 the proportions of its various cleavage products as given in 

 Chapter I (50), so that if casein is formed from the protein of the 

 blood or tissue, a considerable reconstruction of their mole- 

 cules is necessary. On the other hand, if the casein of the milk 

 is built up in the epithelial cells of the udder, in the manner 

 suggested in Chapter V (226, 227), from the simpler cleavage 

 products in the blood, the process is specific for the milk gland. 



553. Origin of milk fats. It was stated in Chapter V (247- 

 249) in discussing the sources of body fat that although the latter 

 may be derived in part from the fat of the feed and show some 

 of its characteristics, nevertheless, the production of fat must 

 be regarded as due essentially to the activity of the fat cells, 

 and not to a simple deposition. 



In the first place, it has been demonstrated by the researches 

 of Jordan and others that milk fat as well as body fat may be 

 formed from the carbohydrates of the feed. 



In Jordan's l experiments cows were fed either with an ordinary 

 ration or with one very poor in fat and the production of fat in the 

 milk determined. After deducting the maximum amounts of fat 

 which could possibly be accounted for by the protein and fat of the 

 feed, a considerable balance was left which could only have been pro- 

 duced from the carbohydrates. The following table gives a summary 

 of the results : 



TABLE 122. PRODUCTION OF FAT BY Cows 



1 N. Y. (Geneva) Expt. Sta., Buls. 132 and 197. 



2 Digested protein of feed less gain of protein by the animal. 



