ENCAPSULATION OF TUBERCULAR FOCI. 271 



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

 tuberculous changes still proceed, both as diffuse casea- 

 tion and formation of miliary tubercles. The whole 

 cavity, with the reactive changes in the tissues of its 

 walls, may be considered as representing a single tubercle, 

 its wall forming a tissue very analogous to the outer 

 zone of the single tubercle, 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 aifected with tubercular consolidation of the 

 lungs, fluid cultures of the streptococcus pyogenes, the 

 result of the mixed infection thus brought about was 

 cavity formation in eight out of nine lungs subjected to 

 the conditious 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 cavities 

 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. 



ENCAPSULATION OF TUBERCULAR Foci. It not 

 uncommonly occurs that round about a necrotic tuber- 



1 Pruddeu : Experimental Phthisis in Eabbits, with the Formation of Cavi- 

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

 ix. p. 166. 



