THE SECRETION OF MILK. 39 



The transudation theory assumes that milk is 

 filtered from the blood. The objections are that 

 neither casein nor milk sugar, which are constituents 

 of milk, are found in the blood ; hence must be a 

 result of cell activity. The fat in the milk is also in 

 much greater proportions than found in the blood ; 

 while the ash constituents of milk and blood have 

 their proportions of soda and potash reversed. If the 

 udder were simply a filter, we should expect milk and 

 blood to be similar in composition. 



The secretion of milk is also ascribed to ferments 

 acting upon the blood in the cow's udder, but this 

 does not seem feasible. 



A combination of the metamorphic, or change, and 

 the transudation, or filter, theories, possibly offers the 

 best explanation. The fat, casein, sugar, and some 

 of the ash constituents of milk are probably the result 

 of cell activity, while the water, albumen, and some 

 other parts are probably filtered from the blood by 

 the glands. 



Milk may be drawn from the udder through the 

 teats after the death of the cow. 



Strippings (last drawn milk) are richer in fat than 

 the first or middle portions of the milking, because 

 the fat globules are retarded in their flow from the 

 vesicles by the walls of the ducts through which the 

 milk flows on its way to the milk-cistern. 



Cows' udders may be classified as " well-balanced," 

 " deficient in front," " poor behind," " deficient in one- 

 quarter," " bottle-shaped," " pendulous," " fleshy," 

 " small," " large," and " elastic." 



