TUBERCULOSIS 283 
and yet remain alive for months, without losing its power of 
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, killit. Unlike many 
bacteria the tuberculosis organism is quite limited as to the condi- 
tions 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 development is possible are quite narrow. It grows best at a 
temperature between 96° and 105°F., but it will grow more slowly 
at a temperature 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 experiments 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 tem- 
perature and a medium equivalent to the blood of such an animal. 
But here, too, bacteriologists 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 
conditions quite different from those of the living body, neverthe- 
less, 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 importance in understanding its distribution. It is true 
