28 MILK SECRETION 



pounds, the whole udder would have to be renewed twice daily. 

 This is not consistent with our pres nt knowledge of tissue 

 building. 



The chief incentive to milk secretion is maternity. As soon 

 as the young mammal is born the blood which went to the 

 uterus to supply the calf is turned toward the udder instead. 



FIG. 3. A schematic figure showing the course of the artery leading to the 

 mammary gland and the veins returning to the heart. The light-colored 

 lines represent arteries and the dark-colored lines the veins. (From Bitting, 

 Twelfth An. Report, Indiana.) 



As soon as this current of blood begins to flow, all of the blood- 

 vessels and capillaries in the cow's udder swell. This causes the 

 minute blood-vessels or capillaries which form a network in the 

 walls of the alveoli to swell, and their swelling stimulates the 

 epithelial cells to activity. 



Conditions Affecting Secretion of Milk. There are a great 

 many conditions which affect the milking capacity of a cow. 

 These conditions may be conveniently grouped into two classes 



