secreted.~ When examined under normal conditions milk
always reveals bacterial life, yet in the secreting cells of the udder
of a healthy cow germ life is not found. Only when the gland is diseased
are bacteria found in any abundance. In the passage of the milk from
the secreting cells to the outside it receives its first infection, so
that when drawn from the animal it generally contains a considerable
number of organisms.

[Illustration: FIG. 5. Microscopic appearance of milk showing relative
size of fat globules and bacteria.]

~Contamination of milk.~ From this time until it is consumed in one form
or another, it is continually subjected to contamination. The major part
of this infection occurs while the milk is on the farm and the degree of
care which is exercised while the product is in the hands of the milk
producer is the determining factor in the course of bacterial changes
involved. This of course does not exclude the possibility of
contamination in the factory, but usually milk is so thoroughly seeded
by the time it reaches the factory that the infection which occurs here
plays a relatively minor role to that which happens earlier. The great
majority of the organisms in milk are in no wise dangerous to health,
but many species are capable of producing various fermentative changes
that injure the quality of the product for butter or cheese. To be able
to control abnormal changes of an undesirable character one must know
the sources of infection which permit of the introduction of these
unwelcome intruders.

~Sources of infection.~ The bacterial life that finds its way into milk
while it is yet on the farm may be traced to several sources, which may
be grouped under the following heads: unclean dairy utensils, fore milk,
coat of animal, and general atmospheric surroundings. The relative
importance of these various factors fluctuates in each individual
instance.

~Dairy utensils.~ Of first importance are the vessels that are used during
milking, and also all storage cans and other dairy utensils that come in
contact with the milk after it is drawn. By unclean utensils, actually
_visible_ dirt need not always be considered, although such material is
often present in cracks and angles of pails and cans. Unless cleansed
with especial care, these are apt to be filled with foul and decomposing
material that suffices to seed thoroughly the milk. Tin utensils are
best. Where made with joints, they should be well f