PHYSIOLOGY OF MILK SECRETION 3 
out of the alveoli. An alveolus which has undergone 
involution and regeneration remains inactive until the 
next parturition. As the period of the secretory activity 
of the udder advances, the number of alveoli in this con- 
dition increase and the quantity of milk secreted con- 
sequently decreases. ‘This change occurs slowly in some 
cows and rapidly in others, and is commonly spoken of 
as “going dry.” Usually the secretion decreases until 
it stops altogether; the cow is then said to be “dry.” As 
the termination of secretory activity in the udder ap- 
proaches, the composition of the milk is considerably 
changed. Secretion of milk is also called “lactation,” 
and the period during’ which a cow produces milk is 
called ‘“‘a lactation period.” 
As the cow advances in age, usually beginning with 
the fifth lactation, some of the worn-out alveoli are not 
regenerated. With each succeeding lactation an increas- 
ing number remain permanently inactive or break down, 
and the quantity of milk produced is consequently de- 
creased. The interalveolar and interlobular connective 
tissue gradually increases in volume, while the gland tis- 
sue decreases. (See Fig. 3.) Finally the udder becomes 
firm and hard and is said to be “fleshy.” A similar 
change is sometimes caused by disease in young as well 
as in old cows. 
Stages of Lactation.—The function of milk secre- 
tion is intimately related to the function of reproduction. 
Pregnancy stimulates the development of the gland tis- 
sue of the udder and secretion begins a few days before 
or at the time of parturition. Why the udder begins 
to secrete at this time is not known. Of the many the- 
ories advanced, that of Schein appears to be the most 
plausible. According to this theory, the blood of the 
