PRIMARY INFECTION. 295 



Encapsulation of Tubercular 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 le&s 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 intercur- 

 rent interference they are caused to break through their 

 envelopes. With the passage of the bacilli or their 

 spores from the central foci into the v^ascular 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- 

 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. 

 Through these channels the bacilli gain access to the 

 tissues and become lodged in the finer capillary ramifi- 

 cations or in the more minute lymph-spaces. Here 

 they find conditions favorable to their development, 

 and in the course of their life-processes produce sub- 

 stances of a chemical nature which act directly in 

 bringing about the death of the tissues in their imme- 

 diate neighborhood. This tissue-death is probably the 

 very first effect of the bacilli in the body, and repre- 

 sents the necrotic centre which can always be seen in 

 even the most minute tubercles. With the production 

 of this progressive necrosis — for progressive it is, as it 



