346 MICRO-ORGANISMS AND DISEASE [cHAP. 



cular from non-tubercular matter, and they have formulated 

 this fact by saying that only matter that is derived from tu- 

 bercle is capable of producing tubercle, and that whenever 

 any substance is found capable of producing this iris tuber- 

 culosis, it is derived from tubercle. By this clear proof for 

 the first time a means was offered to make a definite differ- 

 ential diagnosis between tubercular and non-tubercular 

 matter, a diagnosis which those who, at former times, were 

 engaged in work on tubercle, found extremely difficult. As 

 is well known from clinical observation, the diagnosis of 

 tuberculosis of the lungs is sometimes associated with diffi- 

 culties : the physical examination and symptomatology do 

 not always insure a correct diagnosis. It is true that Ville- 

 min had proved by experiment that tuberculosis is inocu- 

 lable, and Wilson Fox had insisted on the specificity of 

 tuberculosis by numerous experiments which he himself had 

 carried out, yet there were authorities who did not draw this 

 sharp distinction, but were rather inclined to the view that 

 artificial tuberculosis is due to infective matter derived from 

 a variety of sources not necessarily always tubercular. For 

 that period, therefore, the exact proof given by Cohnheim and 

 Salomonson marked a very important step, though the exact 

 nature of this specific tubercular virus remained undeter- 

 mined. The next discovery was that of Koch, who showed 

 what this nature is ; he demonstrated a particular species of 

 bacilli, now familiar to all pathologists as the tubercle-bacillus, 

 which he found only in tubercle and in no other disease, a 

 bacillus so peculiar and so constant that its important dia- 

 gnostic value was at once recognised. No matter whether 

 it is a nodule in any tissue or organ that does or does not 

 present the typical pathological (gross and minute) charac- 

 ters of the classical tubercle, no matter whether in man or 

 the bovine species, in the sheep, in the monkey, dog, cat. 



