24 Agricultural Bacteriology. 



The contamination of milk must be considered from 

 two points of view: (1) the economic, that is the con- 

 tamination with thos'e forms of bacteria that cause the 

 milk to sour, (2) the hygienic, the contamination with 

 those forms of bacteria that produce disease in human 

 beings. Both of these phases of milk contamination 

 are becoming more important each year, as more is 

 learned of the ways in which diseases are spread and as 

 the rapid growth of the cities makes it necessary to 

 draw milk from more distant sources. 



Condition of milk when formed in the udder. When 

 the milk is secreted in the udder of a healthy cow it is 

 sterile. The various internal organs and the blood are 

 practically sterile in healthy animals, hence any sub- 

 stance which is formed from the blood must be sterile. 



Condition of the milk when drawn from the udder. 



If & sample of milk is drawn into a sterile vessel in such 

 a way as to prevent all external contamination, it will 

 be found to contain a greater or less number of bac- 

 teria, which must have come from the udder of the ani- 

 mal. 



The udder is composed of the secreting tissue held 

 in place by the fibrous connective tissue. From the upper 

 part of the glandular secreting tissue, small tubes lead 

 downward, joining each other until they communicate 

 with a small cavity known as the milk cistern, which 

 holds about one half-pint. From this cistern the milk 

 flows into the teat which is guarded by muscles, at the 

 top and bottom. A normal contraction of these muscles 

 keeps the milk from leaking out of the udder. 



The end of the teat inevitably becomes soiled with 

 material containing bacteria. Through the opening of 



