PART III. 
BACTERIA IN DAIRY PRODUCTS. 
CHAPTER XL 
BACTERIA IN MILK. 
In no phase of farm life has bacteriology made such profound 
changes as in dairy methods, changes so great as to amount almost 
to a revolution. Many dairy methods of twenty-five years ago 
have been abandoned and many new ones adopted, chiefly through 
the discoveries of bacteriologists. 
BACTERIA IN MILK WHEN SECRETED. 
Milk, when secreted from the mammary gland of a healthy cow, 
is generally, if not always, free from bacteria. It has been no easy 
matter to demonstrate this fact, since there are bacteria in the milk 
before it leaves the udder. But a sufficient number of careful ex- 
periments have shown that these really come from the outside, 
entering the udder through the milk ducts, and that they do not 
come from the milk glands. 
If the cow is not in perfect health her milk may not be free from 
bacteria. When a cow is suffering from generalized tuberculosis, 
or when she has this disease localized in the udder, her milk, when 
secreted, is sure to contain bacteria. Indeed, any udder infection 
due to bacteria, even a simple inflammation of the mammary gland, 
is likely to result in the contamination of the milk with the bacteria 
which cause the trouble. Milk from a cow suffering from udder 
trouble is no longer pure milk. It may contain tuberculosis bacilli, 
12 137 
