MILK AND ITS PRODUCTS 131 



place in the udder of the cow, and principally during the 

 process of milking. Further, the entire process of milk 

 elaboration seems to be under the control of the nervous 

 system of the cow. This accounts for the changes in flow 

 and richness of milk whenever cows are subjected to 

 abnormal treatment. It is well known that a change of 

 milkers, the use of rough language, or the abuse of cows 

 with dogs and milk stools, seriously affects the production 

 of milk and butter fat. It is therefore of the greatest 

 practical importance to milk producers to treat cows 

 as gently as possible, especially during the process of 

 milking. 



How Secreted. The source from which the milk con- 

 stituents are elaborated is the blood. It must not be sup- 

 posed, however, that all the different constituents already 

 exist in the blood in the form in which we find them in 

 milk, for the blood is practically free from fat, casein, 

 and milk sugar. These substances must then be formed in 

 the cells of the udder from material supplied them by the 

 blood. Thus there are in the udder cells that have the 

 power of secreting fat in a manner similar to that by 

 which the gastric juice is secreted in the stomach. Simi- 

 larly, the formation of lactose is the result of the action 

 of another set of cells whose function is to produce lac- 

 tose. It is believed that the casein is formed from the 

 albumen through the activity of certain other cells. The 

 water, albumen, and soluble ash probably pass directly 

 from the blood into the milk ducts by the process known 

 as osmosis. 



Variations in the Quality of Milk. Milk from dif- 

 ferent sources may vary considerably in composition, 

 particularly in the percentage of butter fat. Even the 



