300 MICRO-ORGANISMS AND DISEASE [cHAP. 



pathogenic for diphtheria ; Loffler had first isolated them by 

 culture on blood-serum, but he, and then Hoffmann, found 

 a morphologically similar bacillus in the normal discharges 

 of the fauces. Now Loffler has shown that, while the 

 former or the " diphtheria bacillus " is pathogenic for ani- 

 mals, the latter or pseudo-diphtheria bacillus is not so ; but 

 this, although not accepted by all, nevertheless corresponds 

 to the facts. 



Roux and Yersin {Annales de r Institut Pasteur, iv., p. 409) 

 state that from simple sore throat, as also from normal throat, 

 the pseudo-diphtheria bacillus was isolated by them, which 

 in morphological and cultural respects is identical with the 

 true diphtheria bacillus, but which is not pathogenic to 

 guinea-pigs. They further conclude that this pseudo-diph- 

 theria bacillus is really the diphtheria bacillus after it has 

 lost its virulence. 



As to the virulence of the diphtheria cultures directly 

 derived from the human diphtheritic secretion or membrane 

 and tested on the guinea-pig (see below), this does not stand 

 in any definite relation to the severity of the human case, 

 for extremely virulent (for the guinea-pig) bacilli may be 

 obtained from mild cases, while from severe or fatal cases 

 bacilli are cultivated which are less virulent for the guinea- 

 pig, inasmuch as of the former the subcutaneous injection 

 of less culture material will produce a fatal result in the 

 guinea-pig than of the latter. Similarly the length of the 

 diphtheria bacilli in the membrane and in the cultures ob- 

 tained from this is no index of their virulence ; as a rule 

 when the membrane contains the diphtheria bacilli in al- 

 most pure culture the great majority are relatively short rods. 



Besides, in true diphtheria of the fauces the diphtheria 

 bacilli can be demonstrated in many cases of fibrinous rhinitis, 

 fibrinous croup, and in diphtheria following scarlatina, but 



