TUBERCULOSIS. 187 



to find the necrotic centres of the tubercles adapted to its 

 growth, and completes its life-cycle with the tissue-cells. 

 It is unusual to find healthy-looking bacilli in the necrotic 

 areas, most of them being observed at the edges of the 

 tubercle, where the nutrition is good. From such edges 

 the bacilli are occasionally picked up by leucocytes and 

 transported through the lymph-spaces, until the phago- 

 cyte falls a prey to its prisoner, dies, and sows the seed 

 of a new tubercle. However, for the spread of tubercle 

 bacilli from place to place phagocytes are not always 

 necessary, for the bacilli seem capable of transportation 

 by streams of lymph alone. 



Notwithstanding the steady advance which takes place 

 in most observed cases of tuberculosis, and the thoroughly 

 comprehensible microscopic explanation of it, many cases 

 of tuberculosis make quite perfect recoveries. 



The periphery of every tubercle is a zone of reaction, 

 with a marked tendency to granulation and organization. 

 If the vital condition is such that through inappro- 

 priate nutriment or through unusually active phago- 

 cytosis the activity of the bacilli is checked or their 

 death is brought about, this tendency to cicatrization is 

 allowed to progress unmolested, and the necrosed mass is 

 soon surrounded with a zone of newly-formed contracting 

 fibrillar tissue, by which it is perfectly isolated. In such 

 isolated masses lime-salts are commonly deposited. Some- 

 times this process is perfected without the destruction of 

 the bacilli, but with their incarceration and inhibition. 

 Such a condition is called latent tuberculosis, and may at 

 any time be the starting-point of a new infection and lead 

 to a fatal termination. 



In 1890, Koch announced some observations upon toxic 

 products of the tubercle bacillus and their relation to the 

 diagnosis and treatment of tuberculosis, which at once 

 aroused an enormous but, unfortunately, a transitory 

 enthusiasm. 



These observations, however, are of capital importance. 

 Koch observed that when guinea-pigs are inoculated 



