BACTERIUM TUBERCULOSIS 409 



tubercles. The whole cavity with the reactive changes in 

 the tissues of its walls may be properly conceived as a single 

 gigantic 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 produce 

 in rabbits conditions that eventuate in 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 strepto- 

 coccus pyogenes into the trachea of rabbits already affected 

 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.' The 

 investigations of Ayer^ not only conflrm the findings of 

 Prudden, but reveal additional facts of very great practical 

 importance. He demonstrates that experimental cavity 

 formation is very largely dependent upon the mass, physi- 

 cally speaking, of tubercle bacilli used for the intratracheal 

 injection; that uncomplicated tubercular infection is not 

 usually accompanied by fever, but that if there be engrafted 

 upon such infection, another type of infection (in Ayer's 

 Experiments, Streptococcus Infection) that fever was ob- 

 served in something over 69 per cent, of the animals used 

 in his investigations. 



' Prudden, Experimental Phthisis in Rabbits, with the Formation of 

 Cavities, etc.. Transactions of the Association of American Physicians, 

 1894, ix, 166. 



2 Journal of Medical Research, November 2, 1914, xxx, 141. 



