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 



