25 



Formation of Milk. 



In spite of the many investigations concerning the formation of milk 

 there is very little positive knowledge on the subject. Previous to 1840 

 it was held that the milk glands separated the materials for the forma- 

 tion of milk directly from the blood. Later it was proved that none of 

 these materials existed ready formed in the blood, so the above theory 

 was abandoned. The second theory was suggested by Virohow, for- 

 mulated by Will, and supported by Viot. They claimed milk to be a 

 fatty degeneration of the epithelial cells of the alveoli, — a liquefied cell 

 substance. Heidenhain opposed this view, and claimed that the epithe- 

 lial cells do not sutfer degeneration, but rather only their free ends, the 

 renewal of the cell material taking place at the opposite end. C Partsch 

 holds that the formation of milk fat is not a result of fatty degeneration, 

 " but rather a special activity of the cell in the true sense of the word." 



Rauber advanced the idea, as a result of his studies, that the fat, casein 

 and milk sugar are derived from the white blood corpuscles. He claims 

 that the lymph rather than the blood vessels are found in direct contact 

 with the alveoli, and that the white blood corpuscles are diffused through 

 the veins and the lymph vessels and then further into the ephithelial 

 cells, and there are transformed into the various milk constituents. 



It is probable that the milk is not gradually formed and stored in the 

 udder during the day, but rather that the epithelial cells of the alveoli 

 become very much enlarged with various substances going to form the 

 milk, and that just previous and even during milking these distended 

 cells break and the milk thus distributed in the udder is drawn off by 

 the milker. The enlarged udder of the cow for a few hours just pre- 

 vious to milking is not wholly due to the amount of milk it contains, 

 but rather to the distended or congested condition of the glands. 



From the above brief outline of the several theories it is apparent that 

 as yet we have but very little definite knowledge of what actually takes 

 place in the glands which results in the production of the white fluid 

 termed milk. It is probable that the fat is derived to some extent from 

 the fat of the blood, but is formed largely as a result of cell activity, and 

 is therefore a secretion. The formation of milk sugar and casein is 

 still largely a matter of conjecture. 



It is evident, however, both from practical and scientific observations, 

 that the production of milk depends firstly upon the large development 

 and great activity of the milk glands, and secondly upon the capacity of 

 the animal to consume and digest large amounts of food in order to sup- 

 ply these glands with an abundance of the necessary raw materials. 



Properties of Milk.* 

 Milk in its normal condition is a pure, almost white fluid, and per- 

 fectly opaque in large quantities. This lack of transparency is due 

 partly to the fat in suspension and partly to a suspension of the nitro- 



• In considering^ this subject it will be understood that cow's milk is always 

 meant, this being practically the only milk used in the United States. 



