246 MICRO-ORGANISMS AND DISEASE. [CHAP. 



instances occasion to verify, before ten to fourteen days have 

 passed. 



One of the most important points, and the most difficult 

 of comprehension, is this power of the pathogenic organisms 

 to resist the influence of the healthy tissues of the living 

 animals, a power which we said above is not possessed by 

 the non-pathogenic organisms. A careful analysis shows at 

 the outset that this power of the pathogenic organisms is 

 not possessed by them indiscriminately, for while a particular 

 species is in some animals capable of overcoming the 

 influence of the living tissue, i.e. to multiply and to produce 

 the particular disease, in other animals it is not capable of 

 doing this, and hence the animal remains unaffected it is 

 said to be not susceptible to the disease. Thus, for instance, 

 the bacillus anthracis when introduced into a human being 

 or a herbivorous animal, is capable of multiplying and of 

 producing anthrax, whereas in carnivorous animals and even 

 in the omnivorous pig it is not capable of doing so. Or 

 again, the bacillus of swine-plague while capable of produc- 

 ing the disease in swine, rabbits, and mice, is not capable of 

 doing so in man, bird, the guinea-pig, or carnivorous animals. 

 Now, where are we to look for this difference in behaviour ? 

 The tissues and juices of a pig when obtained as infusion or 

 otherwise are just as good a nourishing material for the 

 bacillus anthracis as the tissues and juices obtained from a 

 herbivorous animal ; artificial cultures of the former and of 

 the latter behave in exactly the same manner, both as regards 

 copiousness of growth and virulence of the bacilli. Again, 

 artificial cultures of the bacillus of swine-plague made in 

 juice of the tissues of the guinea-pig or fowl are exactly the 

 same as those made of the juice of the tissues of a rabbit 

 or pig. The tissues, therefore, per se cannot be said to possess 

 any inimical action on the organisms. The living condition 



