206 BACTERIOLOGY. 



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 the lower animals 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 cavities 

 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 sometimes be 

 produced by the inoculation of lower animals with the 

 sputum from such cases. 



ENCAPSULATION OF TUBERCULAR Foci. It not un- 

 commonly occurs that round about a necrotic tubercular 

 focus there is formed a fibrous capsule which may com- 

 pletely cut off the diseased from the healthy tissue sur- 

 rounding 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 intercurrent interfer- 

 ence 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 lymphatic circula- 

 tion 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- 

 down of old caseous lymphatic glands is a common 

 example of this condition. 



PRIMARY INFECTION. The primary infection occurs 

 through either the vascular or lymphatic circulation- 



