MILK PRODUCTION 321 



are ultimately transformed into the constituents of milk. 

 The mammary gland is not a sieve through which cer- 

 tain compounds in the blood are strained into the udder 

 cavities, but it is a specialized tissue in which wonderful 

 and extensive chemical changes occur. Here, for the first 

 time, we find casein, the mixture of compounds known 

 as butter-fat, and a sugar unlike any that is found in 

 plants, or in any other part of the animal organism. 

 Vegetable fats contain glycerides similar to some of those 

 found in milk, to be sure, but not in the same number 

 or proportions. One fact, moreover, which dairymen have 

 been slow to recognize in all its significance, is that the 

 udder of each individual cow is a law unto itself in the 

 characteristics of the milk which it secretes, and is not 

 subject in any large degree to control through feeding or 

 other treatment that is not actual abuse. 



The manner of milk secretion is something of which 

 we know but little, and this is, perhaps, not immediately 

 important to the dairyman. The food source of the con- 

 stituents of milk is, on the other hand, a matter of great 

 practical interest, and here we have information more or 

 less definite. 



420. Food sources of milk proteins. The previous 

 discussion of the functions of nutrients must have made 

 it clear that the proteins of the milk can have only one 

 source, viz., the proteins or closely related compounds in 

 the food, a unanimous conclusion which rests upon experi- 

 mental evidence as well as upon the universally accepted 

 truth that the animal organism does not have the power 

 to construct proteins from the simpler compounds used 

 by plants for that purpose. 



421. Food sources of milk-fats. It now seems quite 

 certain that the proteins are the only constituents of 



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