TUBERCULOSIS. 351 



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 equiva- 

 lent to the blood of such an animal. But here, too, bacteri- 

 ologists have changed their views, for the tubercle bacillus 

 will grow now 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 of the body of animals we may 

 look to various places in nature as a source of infection, while 

 if it demands for its existence conditions of the living body, 

 we must look to animals alone as its source. Now although, 

 as noticed, it can grow under conditions quite different from 

 those of the living body, it is nevertheless a fact that, so far as 

 our present knowledge goes, it does not grow outside of the 

 body of animals under any normal conditions. It will not 

 grow in water or in milk, two facts of the utmost importance 

 in understanding its distribution. It is true that the bacillus 

 may frequently be found alive outside of 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 they are normally subjected, and we must therefore 

 conclude that its multiplication is confined to tJie bodies of warm- 

 blooded animals. While it can flourish in the artificial culture 

 media of the laboratory, when kept at special temperatures, it 

 does not flourish in nature outside of the body of warm- 

 blooded animals upon which it lives as a parasite. 



Animals Subject to the Disease. As a parasite it is able to 

 live in a large number of animals. Besides living in man it 

 can flourish readily in the bodies of cattle, hogs, dogs, cats, 

 monkeys, rabbits, guinea-pigs and some other animals. In 

 all these it produces symptoms showing great resemblances, 

 differing, of course, slightly in the different animals. The 



