16 ESSAYS ON BACTERIOLOGY. 



two of these refer to inoculation; and the inoculation 

 experiments are the final and crucial tests of bacteri- 

 ology, at least so far as the pathogenic character of 

 the organisms is concerned. But here a difficulty is 

 at once met. It is evidently impossible to inoculate 

 men with the supposed causes of such diseases a;s 

 cholera, tuberculosis, typhoid, syphilis, anthrax and 

 glanders. Animals must therefore be used, and the 

 question must at once be answered: are animals sus- 

 ceptible to all the infectious diseases? This question 

 resolves itself into two: are they susceptible, and ho^w 

 do the diseases manifest themselves? By what signs 

 and lesions may we know that an animal suffers from 

 or .dies of a certain disease? Are we to expect the 

 same symptoms and lesions as are found in human 

 beings? If not, they must be studied out for aniraala 

 as they have been for man. 



The susceptibility of animals to certain infectioiis 

 is easily recognized; for instance, to erysipelas, suji- 

 puration, tuberculosis, anthrax, glanders and others. 

 Some animals are apparently entirely refractory to 

 one or more diseases to which others are very sensitive. 

 Moreover, it is not sufficient merely to show that inoc- 

 ulation with pure cultures will killan animal; it mky 

 have died of a sapremia or septicemia, not of a specific 

 disease from which the organism was obtained. Fot 

 there are organisms which are not known to have a 

 constant causative relation to any specific disease, 

 which, nevertheless, will produce pronounced toxic 

 effects when inoculated. 



