TEXT-BOOK OF BACTERIOLOGY. 223 



appearance, and the state of things now alters, inasmuch as the 

 rod-cells are also found in the internal organs, where they were at 

 first absent. 



The virulence of the black-leg bacilli is extremely various. In 

 some cases it is subject to natural attenuation. Thus Kitasato 

 found that his bouillon cultures quickly lost their poisonous prop- 

 erty, and often at the end of a week could no longer be transmitted 

 to animals, while they had perfectly preserved their vital energy. 

 With frequent change to fresh food media this loss did not take 

 place, and so far at least no such diminution of infectious power 

 has been noticed with gelatin and agar, not even in very old cul- 

 tures. 



Artificial attenuation is easily produced. Here also heat is the 

 best means ; breeding at 42 or 43 C. robs the bacteria quickly of 

 their pathogenic force. 



It is remarkable that the spores are also susceptible of attenua- 

 tion. If one exposes the flesh of animals which have died of black- 

 leg and which has been dried at about 30 C. to a dry heat between 

 SO and 100 C. for several hours, it gradually loses its virulence. 

 It may even be employed as a means of protective inoculation, and 

 is indeed particularly recommended for practical use, since it pre- 

 serves the diminished degree of virulence unaltered for any length 

 of time. Yet the attenuation, as might be expected from the man- 

 ner in which it is brought about, is not very enduring with the 

 bacteria. The but partially removed virulence may be quickly re- 

 called by mixing up the bacteria (as directed by Arloing, Cornevin, 

 and Thomas) with a 20$ solution of lactic acid and injecting the 

 mixture into susceptible animals. The addition of acid injures the 

 tissue and the vital action of the body, reduces its power of resist- 

 ance, and allows the bacteria to gain a footing. Wheh they have 

 once done this they quickly recover their former power, and after a 

 few more transmissions regain their lost virulence to its full 

 extent. 



In the same position with this striking phenomenon are others 

 noticed chiefly by French investigators, and more particularly by 

 Roger. These observations concern the inoculation of black-leg into 

 animals naturally non-susceptible to it; rabbits, for instance, which 

 at once yield to the malignant oedema, enjoy immunity from the 

 effects of black-leg poison; the same is the case with mice, pigeons, 

 fowls, etc. Yet if one injects them with the mixture of black-leg 

 bacilli already mentioned and solution of lactic acid, or if one also 

 injects at the same time with the black-leg bacilli sterilized or non- 

 sterilized cultures of Micrococcus prodigiosus, Proteus vulgaris, 



