254 BACTERIOLOar. 



the tuberculous 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 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. 



In the contents and in the walls of tubercular cavi- 

 ties bacteria other than the tubercle bacilli are found. 

 It is to the influence of some of these, as we have just 

 seen, that diseases other than tuberculosis may some- 

 times be produced by the inoculation of animals with 

 the sputum from such cases. 



ElSrCAPSFLATIOlSr OF TUBERCULAE Foci. — It not 

 uncommonly occurs that round about a necrotic tuber- 

 cular focus there is formed a fibrous capsule which may 

 completely cut off the diseased from the healthy tissue 

 surrounding it. Or a tubercular focus may, through 

 the resistance of the tissue in which it is located, be 

 more or less completely isolated. In this condition 

 the diseased foci may lie dormant for a long time and 

 give no evidence of their existence, until by some in- 

 tercurrent interference they are caused to break through 

 their envelopes. With the passage of the bacilli or their 

 spores from the central foci into the vascular or lym- 

 phatic circulation the disease may then become general. 



It is to some such accident as this that the sudden 

 appearance of general tubercular infection in subjects 

 supposed to have recovered from the primary local 

 manifestations may often be attributed. The breaking- 



