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 



