THEORIES OF MILK SECRETION 27 



and that the calf received its nourishment from the same source 

 after it was born as it did previous to birth. It was supposed that 

 after the birth of the calf the opening on the uterus through which 

 the food was supplied was closed, and that a new opening was 

 formed in the milk-gland. These two theories have now been 

 practically overthrown. It has been demonstrated that the 

 major portion of the milk is formed within the milk-gland. The 

 fat, casein, milk-sugar, and part of the albumen are supposed to 

 be formed in the udder. This conclusion is substantiated by the 

 fact that these substances do not appear in the blood, at least not 

 to such an extent as to warrant the assumption that they are not 

 manufactured in the cow's udder. The total amount of fat in 

 the blood of the cow would not equal the fat in the milk from one 

 milking. 



By some it is maintained that the substances in milk which 

 are found in solution may be transuded directly from the blood. 

 Here again milk-sugar is found to be in perfect solution in the 

 milk. But this substance can be found nowhere in nature 

 besides in milk. It is not present in the blood of the animal; 

 consequently it must be manufactured within the gland itself. 

 The water of milk, and the ash constituents which are in solu- 

 tion, are probably transuded directly from the blood. No 

 attempts have been made to determine definitely how casein and 

 albumen are formed within the gland. 



According to the theory which has been advanced, the fat is 

 formed by the breaking down of the epithelial cells. When the 

 breaking-down process is completed, the transformed cells appear 

 at the opening of the alveoli in the form of distinct fat-globules. 

 This is supposed to be the origin and formation of fat-globules 

 in milk; so it may be said that so far as known the fat is the 

 result of a breaking down of degenerated epithelial cells. 



Dr. Bitting asserts that the formation of milk solids in the 

 cow's udder is probably due to a metabolic process rather than 

 to a degenerative. Collier found that a cow giving a normal 

 amount of milk would secrete about 136,000,000 fat-globules 

 per second. He also suggests that a cow secretes about 5 pounds 

 of milk solids per day. As a cow's udder weighs only about i\ 



