es.

[Illustration: Fig. 6.--Fat Globules and Bacteria.

Note the relative size of the fat globules of milk and the lactic
acid bacteria.]

=Sources of contamination.= Inasmuch as milk is especially exposed to
the inroads of bacterial growth, and because of the fact that much
of the contamination can easily be prevented, it is highly important
that the milk producer and dealer should be thoroughly cognizant of
the various sources of contamination. The different factors
concerned in contamination may be grouped as follows: the interior
of the udder; utensils, including all apparatus with which the milk
is brought in contact subsequent to withdrawal from the animal;
infection coming from the animal herself, from the milker, and the
surrounding air.

=Condition of milk when secreted.= Immediately after withdrawal from
the udder, milk always contains bacteria, yet in the secreting
cells of the udder of a healthy cow, germ life does not seem to be
present. 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.

A study of the structure of the udder shows the manner in which such
infection occurs.

=Structure of the udder.= The udder is composed of secreting tissue
(_gland cells_) that is supported by fibrous connective tissue. The
milk is elaborated in these cells and is discharged into microscopic
cavities, from whence it flows through the numerous channels (_milk
sinuses_) that ramify through the substance of the udder, until
finally it is conveyed into the _milk cistern_, a common receptacle
holding about one half pint that is located just above the teat.
This cavity is connected with the outside by a direct opening (_milk
duct_) through the teat. During the process of milking, the milk is
elaborated rapidly in the gland cells, and their contents upon
rupture of the milk cells, flow down into the cistern. The normal
contraction of the muscles at the lower opening of the outer duct
prevents the milk from passing out except when pressure is applied,
as in milking. The inner walls of the milk duct and cistern are
always more or less moist, and therefore afford a suitable place for
bacteria to develop, if infection once occurs, and conditions are
favorable for growth.

=Manner of invasion.= Two possible sources