26 



PROFITABLE DAIRYING 



finally terminating in cells. These are the alveoli 

 cells. These cells are about one-thirtieth of an 

 inch in diameter and are arranged in groups of 

 three to five, having a common outlet. The inner 

 walls of alveoli cells are made up of a layer of 

 very minute epithelial cells; sometimes there are 

 two or three layers of these cells. These small 

 cells are filled with protoplasm, and when this 

 protoplasm is discharged it is termed milk. 



The udder not a reser- 

 voir. At one time it was 

 the general belief that the 

 udder was a reservoir, 

 and it may be that many 

 of us still have the impres- 

 sion that this is the case. 

 That this is not true is 

 easily shown by the fact 

 that the total capacity of 

 all the reservoirs or cis- 

 terns is not one-fourth the 

 yield of the milk. 



The glands secrete milk 

 all the time, but princi- 

 pally at the time of milk- 

 ing. We can compare this 

 to the secreting of tears by the tear glands 

 of the eye. These glands secrete tears all the 

 time, but especially when an animal experiences 



Fig\ 3. Cross-section of a 

 cow's udder, showing: cavities 

 and milk cisterns. 



