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ions and gradual the inception and manifestation of the 

 disease, so numerous and diverse the other influences to 

 which the individual is exposed, so impracticable the re- 

 striction of personal liberty necessary for accurate ob- 

 servation, that the exclusion of other possible causes for 

 the disease has not been, and probably cannot be, con- 

 clusively demonstrated in the human subject. Clinical 

 observations have therefore never been decisive, either 

 affirmatively or negatively. With experiments upon ani- 

 mals it is evidently otherwise ; and this question was, 

 early in the history of experimental pathology, submitted 

 to experimental investigation. In 1865 Villemin de- 

 monstrated that the subcutaneous introduction of tuber- 

 culous human tissues was followed by local and general 

 tuberculosis in rabbits and guinea-pigs. His results 

 were in succeeding years corroborated by Klebs, Lebert, 

 Waldenburg, Cohnheim, Frank el, Tappeiner, Orth, Bol- 

 linger in short by all who made the experiment ; yet 

 not every inoculation was successful : the animals most 

 frequently subject to spontaneous tuberculosis es- 

 pecially the rabbit and guinea-pig were found also 

 most susceptible to inoculation ; those which rarely 

 exhibit the disease spontaneously the dog and cat, for 

 example often resisted attempts at artificial induction 

 of the disease ; this was to be expected, and was con- 

 sidered, indeed, as clinical confirmation of the ana- 

 tomical evidence as to the identity of the spontaneous 

 and the induced disease. Tuberculosis can, there- 

 fore, according to the unanimous testimony of ob- 

 servers, be induced by inoculation with tuberculous 

 tissue. But it soon became doubtful whether this un- 

 questioned fact could be interpreted as proof that there 

 is anything specific about the tubercle ; for it is evident 

 that if all the effects produced by inoculation with tubercle 

 can be just as certainly induced by non-tuberculous 

 materials, no assumption of specific nature is necessary. 

 It was demonstrated by Burdon-Sanderson, Wilson Fox, 

 Martin, Waldenburg, Cohnheim, Frank el, that after the 

 introduction of mechanical or chemical irritants a piece 



