MILK SECRETION 



27 



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 



Fig. 3. — A schematic figure showing the course of the artery leading to the 

 mammary gland and the veins returning to the heart. The light-colored 

 lines represent arteries and the dark-colored lines the veins. (From 

 Bitting, Twelfth An. Report, Indiana.) 



to a dcgencrati^-e. 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 2|- pounds, the whole udder would have to be renewed 

 twice daily. This is not consistent with our present knowledge 

 of tissue building. 



