Illustration: FIG. 8. Sectional view of udder showing teat with milk
duct connecting exterior with the milk cistern. Milk sinuses are mostly
shown in cross section interspersed and below the secreting tissue
(Moore and Ward).]

The udder is composed of secreting tissue (_gland cells_) held in place
by fibrous connective tissue. Ramifying throughout this glandular
structure are numerous channels (_milk sinuses_) that serve to convey
the milk from the cells where it is produced into the _milk cistern_, a
common receptacle just above the teats. This cavity is connected with
the exterior through the milk duct in the teat, which is more or less
tightly closed by the circular sphincter muscles, thus preventing the
milk from flowing out. The mucous membranes of the milk duct and cistern
are naturally moist. The habits of the animal render it impossible to
prevent infection of the external opening at the end of the teat and
there is no mechanical reason why bacteria cannot readily find their way
along the moist lining membrane for some distance. If organisms are
adapted to this kind of an environment, ideal conditions exist for their
multiplication, as moisture, warmth and suitable food supply are
present. The question arises how far up into this organ is penetration
possible? Within late years numerous observations have been made on the
presence of organisms in the upper portion of the udder in contact with
the secreting tissue.[8]

These investigations prove that bacteria are distributed throughout the
whole of the udder, although numerically they are much less abundant in
the region of the milk-secreting tissue than in the lower portion.
Ward's conclusions are "that milk when secreted by the glands of a
healthy udder is sterile. It may however, immediately become
contaminated by the bacteria which are normally present in the smaller
milk ducts of the udder."

~Nature of bacteria in fore milk.~ Generally speaking the number of
different species found in the fore milk is not large, and of those
which do appear many occur at only occasional intervals. Moore[9] in the
examination of 9 udders found 20 different forms, and of these only 3
species predominated, all of which proved to be micrococci. Streptococci
have also been quite frequently reported. Freudenreich[10] found the
most common types to be cocci, belonging to both the liquefying and
non-liquefying class.

Peptonizing[11] and spore-bearing[12] species have also been