THE RELATION OF MICROORGANISMS TO MILK. 297 



SOURCES OF MICROORGANISMS IN MILK. 



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 



