THE RELATION OF MICROORGANISMS TO MILK. 

 SOURCES OF MICROORGANISMS IN MILK. 



297 



The sources from which bacteria get into the milk have been the sub- 

 ject of much investigation during the past few years, until now the chief 

 sources of contamination are pretty well understood. These sources 

 may be grouped in a general way under the following heads: 



FIG. 73. Vertical section of one quarter of udder showing teat, milk cistern, and larger 

 milk ducts. (After Ward and Hopkins.) 



INTERIOR OF THE Cow's UDDER. Healthy Udders. Milk as it is 

 secreted by the normal udder of a healthy cow is comparatively free from 

 bacteria. It is very difficult, however, to obtain milk from the udder which 



