68 STORRS AGRICULTURAL EXPERIMENT STATION. 
or ferment, but may be kept indefinitely without any change 
taking place that is noticeable. , 
Second, all the ordinary changes which occur in milk after 
being drawn from the cow are due to the presence of bacteria 
in the milk. This includes not only the common souring, but 
all of the other changes which occur at intervals to trouble 
the dairymen. 
Third, the bacteria which produce these changes are all 
secondary contaminations and do not belong to the milk as 
' secreted from the gland provided the gland be healthy. 
Fourth, the sources from which the bacteria that contami- 
nate the milk are derived are several, At the head stands the 
cow herself. Bacteria lurk in the milk ducts. They adhere 
to the outside of the animal, clinging to the hairs, and, during 
the milking, find their way into the milk in great abundance. 
Next, the vessels in which the milk is drawn and in which it 
is kept are almost never clean, and contain bacteria in great 
numbers, ready to grow as soon as milk is placed in the vessel. 
Again, the dust of the air in the milking stall is a source of con- 
tamination. The extent of contamination from this source 
will vary widely and will be especially abundant when the milk 
stall is full of the dust from newly-disturbed hay. Lastly, the 
milker himself, his hands and his clothing, are sources of con- 
tamination. | 
Fifth, the action of the bacteria upon milk in producing 
undesirable changes is dependent upon temperature, for the 
bacteria in question grow slightly, if at all, at temperatures 
near freezing, and grow rapidly at warmer temperatures. 
A second series of facts of perhaps even greater importance 
has been discovered in connection with the study of milk as a 
distributor of disease. The past ten years has shown beyond 
peradventure that milk is a prolific means by which certain 
diseases are distributed in man. We have learned that milk is, 
at the same time man’s best food as determined by its chem- 
istry and by its ease of digestion and assimilation, and the 
most dangerous food, from the fact that when improperly 
handled it may be the means of distributing disease to man- 
kind. 
The diseases which we have learned in these few years are 
distributed by milk are, however, not numerous. They are 
as follows: Tuberculosis, which comes directly from animals 
suffering from the disease and may be under special circum- 
stances transmitted to man; diphtheria and_ scarlet fever, . 
