THE TUBERCLE BACILLUS 301 



After caseation has occurred calcification may ensue 

 that is, lime-salts are deposited and the nodule is converted 

 into a calcareous mass. 



So far back as 1865 Villemin showed that inoculation 

 of rabbits with human caseous material was followed by a 

 development of nodules similar in all respects to the miliary 

 tubercles in man. Cohnheim, Burdon Sanderson, and 

 Wilson Fox confirmed this observation, but they also 

 showed that the development of tubercles apparently 

 followed the introduction, not only of tuberculous material, 

 but also of setons, pieces of putrid muscle, and gutta- 

 percha. It was pointed out, however, that in all proba- 

 bility these results were due to accidental contamination 

 or inoculation with tuberculous matter, and, by adopting 

 suitable precautions in order to prevent such sources of 

 error, it was conclusively shown that non-tuberculous 

 matter is unable to set up tuberculosis. Tuberculosis is 

 therefore inoculable, and is an infective disease, and as 

 such must be due to a specific infective agent, to the 

 discovery of which observers then directed their attention. 

 In 1882 Koch announced that he had discovered a special 

 bacillus, the tubercle bacillus, in tuberculous tissues, which 

 could be isolated and cultivated, and which reproduced the 

 disease on inoculation. 



The Tubercle Bacillus 



Morphology. The tubercle bacillus (B. tuberculosis) is 

 a slender rod with rounded ends, often slightly curved, and 

 averaging 2-3 /m in length, though the length varies in 

 the tissues from 1*25 /JL to 6'5 JUL ; in cultures it tends to be 

 short, on serum being about 1 yu. In stained preparations 

 one or more unstained intervals are often seen in the rods 

 (Plate VIII. a) ; these have been considered by some 

 observers to be spores, but there are many objections to 



