282 BACILLI PATHOGENIC IN MAN. 



grow better there than the others, and hence, in all 

 probability, we shall see one or other species in largest 

 numbers in the great majority of cases, thus leading us 

 to the erroneous conclusion that they have a specific 

 etiological meaning. It is evident that here we can only 

 attempt to isolate the true infective agent with extreme 

 caution, and with care that fall weight is attached to 

 all the sources of error, 

 insuscepti- Further difficulties arise from the behaviour of the 



bility of the , ,. .. IIP . mi 



lower animals, animals ordinarily employed for experiments. These 

 are much less sensitive than man to the infective agents 

 of diphtheria ; infective experiments with diphtheritic 

 membrane have been carried out very extensively in 

 various kinds of animals, but very often without 

 any corresponding effect, although the membranes were 

 placed directly on the mucous membrane of the open 

 trachea (Trendelenburg, Francotte, and others). In 

 some cases it is true that illness of the animals, for- 

 mation of false membranes in the trachea, &c., have 

 been observed, but there has never been a development 

 of typical diphtheria, with all its various symptoms, and 

 those 'morbid phenomena which were observed could 

 also be caused by inoculation with non-diphtheritic 

 putrefying material (Hueter, Marcuse, and others), and 

 also with various species of bacteria evidently not in 

 etiological connection with human diphtheria, but never- 

 theless occurring at times in the normal secretions of 

 the mouth. Such morbid conditions in animals re- 

 sembling diphtheria cannot be of themselves utilised 

 for the recognition of the infective agent of human 

 diphtheria, and thus there is increased difficulty in 

 experiments made for the purpose of clearing up the 

 etiology of this devastating disease. 



Various kinds Finally, many clinical and epidemiological facts indi- 

 of diphtheria. cate ^^ ^^ are varioug forms O f diphtheria caused 



by different infective agents. If this idea is confirmed 

 it is evident that a further complication of the investiga- 

 tions is unavoidable. 



Former investigators have, without doubt, paid too 

 little attention to these dangers and difficulties, and the 



