138 BACTERIA IN MILK. 
or, in cases of inflamed udders, it is likely to contain pus, together 
with considerable quantities of chain-forming streptococci. These 
should not be present in good milk, and there is reason for believing 
that they are the cause of certain illnesses in man. 
Confining our attention for the present to milk from healthy 
animals, we notice that, if we could keep bacteria out of the milk, 
none of the ordinary changes, not even the souring which is so 
nearly universal in normal milk, would take place. Indeed, milk 
which is free from bacteria will remain visibly unchanged for an 
indefinite time. It is not, however, absolutely free from subsequent 
chemical changes, since there is present in the milk an enzyme which 
produces slow changes. This enzyme, called galactase, is secreted 
by the milk gland with the milk, and may thus be said to be part of 
the milk. It can slowly convert the casein of the milk into soluble 
proteids. Its action is very slow, however, and seemingly of no 
significance, except in the ripening of cheese. At all events, none 
of the ordinary fermentations appearing in milk are attributed to 
this galactase or to any other part of the milk itself, but are all due 
to microorganisms. We may, therefore, take as a starting-point 
these two highly important facts: i. Milk from healthy cows will, 
if it could be kept free from bacteria, show none of the ordinary 
milk fermentations. 2. All of these fermentations are due to 
microorganisms that get into the milk after the milk is secreted from 
the mammary gland. 
SOURCES OF MILK BACTERIA. 
Recognizing that milk is germ free when secreted from the milk 
gland, we are hardly prepared to learn that, by the time it has been 
drawn from the cow, received in the milk-pail, and removed from the 
cow stall, it may contain bacteria to the extent of many thousands 
per c.c. But this is frequently and, indeed, commonly the case. 
The number -of bacteria in freshly drawn milk varies greatly with 
the conditions existing in the dairy. There may be only a few 
hundreds in each c.c., or, under exceptional conditions, a smaller 
number still; but it is much more likely that the milk, by the time 
