400 BACTERIOLOGY. 



Under certain circumstances with which we are 

 not acquainted bacillus diphtherice becomes diminished 

 in virulence or may lose it entirely, so that it is no 

 longer capable of producing death of susceptible ani- 

 mals, and may cause only a transient local reaction 

 from which the animal entirely recovers. Sometimes 

 this reaction is so slight as to be overlooked, and 

 again careful search may fail to reveal evidence of 

 any reaction at all. This exhibition of the extremes of 

 its pathogenic properties, viz., death of the animal, on 

 the one hand, and only very slight local effects on the 

 other, was at one time thought to indicate the existence 

 of two separate and distinct organisms that were alike 

 in cultural and morphological peculiarities, but which 

 differed in their disease-producing power. Further 

 studies on this point have, however, shown that the 

 genuine bacillus diphtherice may possess almost all 

 grades of virulence, and that absence of or dimi- 

 nution in virulence can hardly serve to distinguish 

 as separate species those varieties that are otherwise 

 alike ; moreover, the histological conditions found at the 

 site of inoculation in animals that have not succumbed, 

 but in which only the local reaction has appeared, are 

 in most cases characterized by the same changes that 

 are seen at autopsy in animals in which inoculation has 

 proved fatal. 



In the course of their observations upon a large 

 number of cases Roux and Yersin found that it was 

 not difficult to detect, in the diphtheritic deposits of 

 the same individual, bacilli of identical cultural and 

 morphological peculiarities, but of very different degrees 

 of virulence, and that with the progress of the disease 



