PROPERTIES OF MILK 15 
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 
