of invasion of the udder by
bacteria may exist. If bacteria are present in the circulating
blood, there is the possibility of organisms passing directly
through the tissues into the milk-secreting cells. The other
alternative is the possible direct contamination from the outside by
organisms passing up through the milk duct, and so spreading through
the open channels in the udder.

[Illustration: Fig. 7.--Sectional View of Udder.

Teat with milk duct connecting the exterior with the milk cistern.
Milk sinuses which conduct the milk from the secreting tissue to the
milk cistern. (After Moore & Ward.)]

=Number of bacteria in fore-milk.= If a bacteriological examination is
made of the milk drawn from each teat at different periods during
the milking process, it will be found that the fore-milk, _i.e._,
the first few streams, contains, as a rule, many more organisms per
cubic centimeter than that removed later. Not infrequently thousands
of organisms per cubic centimeter may be found in the first streams
while the middle milk, or strippings, will contain much smaller
numbers.

=Distribution and nature of bacteria in udder.= If the udder itself is
carefully examined as to its bacterial content, it appears that the
majority of organisms found is confined to the lower portion of this
organ, in the teat, milk-cistern and large milk-ducts; while
bacteria occur in contact with the secreting tissue, they are
relatively less abundant. This would seem to indicate that the more
probable mode of infection is through the open teat.

While there is no constant type of bacteria found in the fore-milk,
yet it is noteworthy that nearly all observers agree that the
organisms most commonly found are not usually the acid-producing, or
gas-generating type, so abundant on the skin or hairy coat of the
udder and which predominate in ordinary milks. Coccus forms,
belonging to both liquefying and non-liquefying types are most
generally present. Many of these produce acid slowly and in small
quantities.

The bacteria coming from the interior of the udder are of small
practical significance since they do not grow rapidly at the
temperatures at which milk is stored. If the milk is protected from
contamination from other sources, the bacteria from the udder will
ultimately cause it to spoil, but under ordinary conditions other
forms are present in such greater numbers, and grow so much more
rapidly in milk, that the udder forms have small o