Distribution of Bacteria. 67 
or the wall cell thickens and = eneloses the  proto- 
plasm. In the spore condition the )acteria are in- 
active, but are able to endure much ereater extremes 
of heat, cold or moisture than when active. When 
the conditions of growth become again favorable, the 
spore again becomes active, or is said to germinate, 
and the vital processes are rested. 
Bacteria are widely distributed through nature. 
In fact, there are very few places where they may 
not be found. They are so light and small that 
they float readily in the atmosphere, particularly when 
accompanied by particles of dust. They are found 
in all rivers and streams, upon the surface of the 
earth, aud upon all organic matter. In fact, they 
are universally distributed. By fav the larger num- 
ber are not only perfectly Jiarmless but positively 
beneficial. They serve to transform dead organic 
matter into its original condition, and so act as 
scavengers. Others, like the milk ferments, bring 
about specific changes in some definite substances, 
while still others, a large class, are the specific 
causes of various diseases in men and animals. 
Presence of bacteria in milk.—In ordinary milk, 
bacteria are always present in large numbers. These 
gain access to the milk from the atmosphere, from 
the bodies of the animal and the milker, through 
contact with the vessels into which the milk is 
drawn, aud to some extent through the udder of the 
animal. The milk when secreted is sterile. So 
far as is known, no bacteria can pass through the 
digestive organs and blood vessels of the animal 
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