ds in the stirring up of the same
increases the number of organisms to be found in the air (Fig. 11).
Thus, the feeding of dry fodder and the bedding of animals with straw
adds greatly to the germ life floating in the air. Dust may vary much in
its germ content depending upon its origin. Fraser found the dust from
corn meal to contain only about one-sixth to one-eighth as much germ
life as that from hay or bran.[36] In time most of these dust particles
settle to the floor, but where the herd is kept in the barn, the
constant movement of the animals keeps these particles more or less in
motion. Much can be done by forethought to lessen the germ content of
stables. Feeding dry feed should not be done until after milking.[37] In
some of the better sanitary dairies, it is customary to have a special
milking room that is arranged with special reference to the elimination
of all dust. In this way this source of infection may be quite obviated
as the air of a clean, still room is relatively free from bacteria,
especially if the floor is moistened. It has often been noted that the
milk of stall-fed animals does not keep as well as that milked out of
doors, a condition in part attributable to the lessened contamination.

[Illustration: FIG. 11. Effect of contaminated air. The number of spots
indicate the colonies that have developed from the bacteria which fell
in 30 seconds on the surface of the gelatin plate (3 inches in
diameter). This exposure was made at time the cows were fed.]

~Relative importance of different sources of infection.~ It is impossible
to measure accurately the influence of the different sources of
infection as these are continually subject to modification in each and
every case. As a general rule, however, where milk is drawn and handled
without any special care, the utensils and the animal contribute the
larger proportion of dirt and bacteria that find their way into milk.
Where the manner of milking and handling is designed to exclude the
largest number of organisms possible, the bacteria appearing in the fore
milk make up the major portion remaining. By putting into practice the
various suggestions that have been made with reference to diminishing
the bacterial content of milk, it is possible to greatly reduce the
number of organisms found therein, and at the same time materially
improve the keeping quality of the milk. Backhaus[38] estimates that
the germ life in milk can be easily reduced to one-two t