294 BACTERIOLOGY. 



miliary tubercles. The material softens and is ex- 

 pelled, and a cavity remains. In the wall of this 

 cavity the tuberculous changes still proceed, both as 

 diffuse caseation and formation of miliary tuber- 

 cles. The whole cavity with the reactive changes 

 in the tissues of its walls may be considered as rep- 

 resenting a single tubercle, its wall forming a tissue 

 very analogous to the outer zone of the single tuber- 

 cle, the cavity itself corresponding to the caseous 

 centre. 



In animals used for experiment cavity-formation of 

 this sort is very rare, owing to the greater resistance of 

 the caseous tissue. That it is, however, possible to pro- 

 duce in rabbits pulmonary cavities in all physical re- 

 spects similar to those seen in the human being has 

 been most beautifully demonstrated by Prudden. He 

 showed that when he had injected into the trachea of 

 rabbits, already affected with tubercular consolidation 

 of the lungs, fluid cultures of streptococcus pyogenes, 

 the result of the mixed infection thus brought about was 

 cavity-formation in eight out of nine lungs subjected 

 to the conditions of the experiment; while in only one 

 out of eleven did cavities form under the influence of 

 the tubercle bacillus alone. 1 



In the contents and in the walls of tubercular cavi- 

 ties in man bacteria other than the tubercle bacillus are 

 found. It is to the influence of some of these, as we 

 have seen, that diseases other than tuberculosis may 

 sometimes be produced by the inoculation of animals 

 with the sputum from such cases. 



1 Prudden : Experimental Phthisis in Rabbits, with the Formation of Cavi- 

 ties, etc. Transactions of the Association of American Physicians, 1894, vol. 

 ix. p. 166. 



