178 MICRO-ORGANISMS AND DISEASE. [CHAP. 



or mucous membranes, subcutaneous tissue, lungs, alimentary 

 canal) and the production of the new brood large enough to 

 produce a definite effect locally or generally, corresponds to 

 the incubation-period of the disease, and, as is well known, 

 there is in this respect a great difference in the different 

 diseases. Thus in anthrax the introduction of the bacilli into 

 the subcutaneous tissue of a suitable animal is followed after 

 from sixteen to twenty -four hours or more by a local effect 

 (oadematous swelling), and a few hours after by general consti- 

 tutional illness, when bacilli can as a rule be found in the 

 blood. On the other hand, in tuberculosis after the introduc- 

 tion of the bacilli tuberculosis into the subcutaneous tissue, 

 the nearest lymph-glands show the first signs of swelling and 

 inflammation after one week or even later, and the general 

 disease of the internal viscera does not follow until one, two, or 

 more weeks have elapsed. This is also borne out by observa- 

 tions of the behaviour of these bacilli in artificial cultures ; 

 whereas a suitable material inoculated with the bacillus 

 anthracis and kept at the temperature of the animal body 

 (38 to 39 C.), shows already after twenty-four hours a good 

 crop of the bacilli ; in the case of the tubercle-bacilli the first 

 signs of a new brood are not noticed, as Koch has pointed out, 

 and as I have had in several 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 multiply- 

 ing 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 



