112 Milk and Its Products 
and appear in the milk. If the udder is the seat 
of disease due to the growth of bacteria, such bac- 
teria may find their way into the milk ducts and 
infect the milk. In one other way the animal may 
be said to be a source of infection with bacteria. 
The end of the teat of the animal is always more 
or less moist. Bacteria coming in contact with 
such surface, moistened with milk, find there not 
only food in proper form for their growth, but a 
temperature sufficient to make them active. They 
begin to multiply, and, working their way through 
the orifice of the teat, find milk in larger supply, and 
a temperature still more favorable for their growth. 
They increase and multiply, under such conditions, 
with remarkable rapidity, and so work their way 
upward through the milk cistern and into the larger 
milk ducts, so that the milk first drawn from the 
animal always contains a greater or less number of 
bacteria. For this reason it is not an easy matter 
to secure perfectly sterile milk direct from the cow, 
though with great care in disinfecting the udder 
and removing the larger part of the milk from it, 
perfectly sterile milk has been obtained. 
Kinds of bacteria in milk.—Almost any of the 
known forms of bacteria may live and grow and oc- 
casionally be found in milk. Normally, however, com- 
paratively few forms of bacteria are present. The 
greater part of these are forms which cause various 
changes in the constituents of milk, and are known 
as ferments, and the changes which they induce as 
fermentations. Beside_these fermentations, there’ may 
