^ XIII.] Bacillus of tubercle. 153 



animals or injected into a blood-vessel or into a cavity of the 

 body, or pure Bacillus-material inhaled in a state of fine division 

 suspended in water, resulted in the formation of tubercle with 

 its consequences in every case without exception — Koch 

 experimented on 217 individuals of susceptible species of 

 animals (rabbits, guinea-pigs, cats, field-mice), besides animals 

 used in control-experiments and individuals of less susceptible 

 species — and the Bacillus was in every case found in the tuber- 

 cles. In every case also the place where the tubercle appeared, 

 its frequency and distribution, and the line of distribution through 

 the body answered the expectations founded on the mode of 

 infection and the spot where the infecting matter was applied. 

 These results, still further strengthened by control-experiments, 

 estabhshed the infectious nature of tuberculosis, and the conta- 

 gious character of the Bacillus, which had indeed been previously 

 concluded on other grounds. 



Such observations on the Bacillus itself as have been published 

 leave much to be desired as regards its morphology. Observers 

 have been generally satisfied with proving its presence, and the 

 proof has been rendered easy by its peculiar behaviour with 

 aniline colouring matters. In contrast to the great majority of 

 other known Bacteria, it slowly and with difficulty takes in alka- 

 line solution of methylene blue or saturated solution of methyl- 

 violet, absorbing them only in the space of several hours or 

 after being heated, but it obstinately retains the colour it has 

 acquired, while other Bacteria are quickly decolorised by certain 

 reagents, for example, dilute nitric acid. By this peculiarity 

 in their behaviour as well as by their shape and size, the Bacilli 

 are comparatively easy to recognise and distinguish from other 

 species. In form they are slender rods which are sometimes 

 curved or bent at an angle, and reach a length of i'5-3-5f. 

 Neither in the natural nor in the coloured state can transverse 

 septation as a rule be observed. Endogenetic spores are found 

 in them, both in cultures and in the body and sputa of diseased 

 animals ; these according to Koch's brief account must answer 



