262 TUBERCULOSIS. 
growth. It will withstand the heat of 140 for fifteen minutes or 
more, and under some conditions a considerable higher heat. A few 
minutes' heating at 175 will, however, kill it. Unlike many bacteria 
the tuberculosis organism is quite limited as to the conditions under 
which it can grow, and an understanding of these conditions is of 
the greatest importance in comprehending the problems of its 
distribution. The temperature limits within which its develop- 
ment is possible are quite narrow. It grows best at a temperature 
between 96 and 105 F., but it will grow more slowly at a tempera- 
ture as low as 84 F. Below this it will not multiply at all. At 
first it was supposed that it would not grow in any artificial medium 
which could be prepared in the laboratory. In his original experi- 
ments Koch was obliged to use coagulated blood serum as a culture 
medium. It is now found that it can live and flourish in a variety 
of culture media, provided a certain amount of glycerin be added. 
It was at first said to be a perfect parasite, by which term is meant 
that it would not live under any conditions except those of a warm- 
blooded animal, demanding both a temperature and a medium 
equivalent to the blood of such an anmial. But here, too, bacteriolo- 
gists have changed their views, for the tubercle bacillus will grow 
in many laboratory media and under conditions very different 
from those of the living body. 
The facts just enumerated are of the greatest significance as 
indicating the possibilities of distribution of this disease. If the 
bacillus can live outside the bodies of animals, we may look to 
various places in nature as a source of infection, but if it demands 
for its existence conditions of the living body, we may look to 
animals alone for its source. Now, although it can grow under con- 
ditions quite different from those of the living body, nevertheless, so 
far as our present knowledge goes, the tubercle organism does not 
grow outside the bodies of animals under any normal conditions. It 
does not grow in water or in milk, two facts of the utmost im- 
portance in understanding its distribution. It is true that the 
bacillus may frequently be found alive outside the bodies of animals. 
It occurs in sputum, in milk, in water, in dust, etc., but in these 
media it does not multiply, at least under any conditions to which 
