306 The Feeding of Animals 



tion to nutrition, because the food expense of milk is 

 determined, other things being equal, not by the vol- 

 ume but by the milk solids elaborated, for which 

 reason the draft upon the supply of nutrients, water 

 excepted, is greater for the secretion of 100 quarts 

 of Jersey milk than for the same quantity of Holsteiri 

 milk. In studying the economy of milk production, 

 therefore, we should consider the relation of food to 

 milk solids and not to milk volume. 



MILK SECRETION ' 



There is no milk in an animal's food, that is to 

 say, hay and grain contain no casein, butter fat or 

 milk sugar. They do contain nutrients, which, when 

 subjected to the vital processes of the animal, 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 wonder- 

 ful 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 de- 



