LOCAL OR GENERAL TUBERCULOSIS. 351 



cles. The material softens and is expelled, and a 

 cavity remains. In the wall of this cavity the tuber- 

 culous changes still proceed, both as diffuse caseation 

 and formation of miliary tubercles. The whole cavity 

 with the reactive changes in the tissues of its walls 

 may be properly conceived as a single gigantic tuber- 

 cle, its Avail forming a tissue very analogous to the outer 

 zone of the single tubercle, the cavity itself correspond- 

 ing 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 

 produce in rabbits pulmonary cavities in all physical 

 respects similar to those seen in the human being 

 has been beautifully demonstrated by Prudden. He 

 showed that when he had injected fluid cultures of strep- 

 tococcus pyogenes into the trachea of rabbits already af- 

 fected with tubercular consolidation of the lungs, 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 bacillus tuberculosis 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; and it is also to the 

 absorption of their toxic products that some of the con- 



1 Prudden : " Experimental Phthisis in Rabbits, with the Formation 

 of Cavities/' etc., Transactions of the Association of American Physi- 

 cians, 1894, vol. ix. p. 166. 



