372 DIPHTHERIA. 



yet the true nature is shown not only by their microscopical 

 characters, etc., but also by the fact that on more prolonged 

 growth they form small quantities of toxine, which is 

 neutralised by diphtheria antitoxine. Neisser also, as the 

 result of an extended enquiry, comes to a similar conclusion 

 with regard to the virulence, and considers that the 

 characteristic staining, the morphological characters, and 

 the production of acid in glucose broth, when taken 

 together, afford conclusive evidence as to the identity of 

 the diphtheria bacillus. 



The question, however, has a special interest in regard 

 to the origin and spread of the disease. As is well known, 

 the disease usually spreads by infection, direct or indirect, 

 from patient to patient ; but sometimes it appears to start 

 afresh, as it were. In the latter case the existence of the 

 non-virulent diphtheria bacilli may afford an explanation of 

 the occurrence, as such bacilli are frequently found even in 

 healthy subjects. For example, Roux and Yersin found 

 the pseudo-diphtheria bacillus in the throats of twenty-six 

 out of fifty-nine children examined, living in healthy sur- 

 roundings. If, accordingly, it may become virulent under 

 certain circumstances, this may explain the occurrence of 

 fresh outbreaks. At present, however, we do not know 

 definitely that such is the case, still less do we know con- 

 ditions which render it virulent. 



Diphtheria does not affect the lower animals, with the 

 exception of cats, which have sometimes been observed to 

 suffer from a similar disease, in association with human 

 epidemics. Klein has found the diphtheria bacillus in the 

 throat of cats in such circumstances. The so-called diph- 

 theria of pigeons, calves, and other animals is produced by 

 entirely different organisms. 



The term xerosis bacillus has been given to an organism first 

 observed by Kuschbert and Neisser in xerosis of the conjunctiva, and 

 which has been since found in many other affections of the con- 

 junctiva and even in normal conditions. Morphologically it is practi- 

 cally similar to the diphtheria bacillus, and even in cultures presents 

 very minor differences. It is, however, non-virulent to animals, and, 



