6 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
Phases of Milk Secretion.—All of the milk obtained 
from the udder at one milking is not secreted before 
the withdrawal of the milk is begun. The average vol- 
ume of the cavities of the udder is 3,000 c.c., or about 3 
quarts, while the average yield of milk at one milking 
is from 4,000 to 6,000 c.c., or about 4 to 6 quarts (F leisch- 
mann). A large part of the milk obtained at a milk- 
ing is secreted while the cow is being milked. As much 
milk is secreted in ten to fifteen minutes during milking 
as is formed during the entire period between milkings. 
The secretion of milk may be divided into two phases 
(Zietzschmann). 
The first phase occurs during the intervals between 
milkings. Following the completion of a milking, the 
gland remains at rest for a short time until the exhausted 
secretory cells can recuperate; then secretion is resumed, 
and gradually increases in intensity up to a certain point. 
The udder gradually becomes larger and firmer, but the 
teats remain relaxed and pendulous and contain only a 
small quantity of milk. The greater part of the milk 
secreted during the first phase remains in the alveoli 
and the large milk canals. There are two reasons for 
this: (1) the horizontal direction of the large milk canals 
does not favor the flow of milk into the cistern, and (2) 
the inward pressure of the erectile tissue in the teat causes 
the mucous membrane to project into the cistern in folds, 
filling up the cavity and opposing the flow of milk into 
it. When the alveoli and the milk ducts and canals be- 
come full, secretion decreases in intensity. Under ordi- 
nary conditions the pressure in the alveoli, ducts, and 
canals does not become sufficient to overcome the coun- 
terpressure exerted by the erectile tissue in the teats, 
