674 Tuberculosis 



framework whose fibers are more resistant to necrosis than the 

 cells, after the cells of a tubercle have been destroyed, fibers may 

 still be visible among the granules, and give the tubercle a reticulated 

 appearance. 



As a rule, tubercles progressively increase in size by the inva- 

 sion of fresh tissue. The tubercle bacilli are usually observed in 

 greatest number at the edges, among the healthy cells, where the 

 nutrition is good. From this position they are swept along by 

 currents of lymph or occasionally are picked up by leukocytes and 

 transported through the lymph-spaces, until the phagocyte falls a 

 prey to its prisoner, dies, and sows the seed of a new tubercle. It is 

 by such continuous invasion of new tissue, the formation of necrotic 

 areas in the lungs, and evacuation through the air-tubes that cavities 

 are formed. In pulmonary tuberculosis the process of destruction 

 is greatly accelerated by inspired saprophytic bacteria that live in 

 the necrotic tissue. The patient also suffers from secondary infec- 

 tions, especially by the streptococcus and pneumococcus. 



If the vital condition of the individual becomes so changed that 

 the invasive activity of the bacilli is checked or their death brought 

 about, the tubercle begins to cicatrize, and becomes surrounded by 

 a zone of newly formed contracting fibrillar tissue, by which it is 

 circumscribed and isolated. This constitutes recovery from 

 tuberculosis. Sometimes the process of repair is accomplished 

 without the destruction of the bacilli, which are incarcerated and 

 retained. Such a condition is called latent tuberculosis, and may at a 

 future time be the starting-point of a new infection. 



Virulence. — The virulence of tubercle bacilli varies considerably 

 according to the sources from which they are obtained. Bacilli 

 from different cases are of different degrees of virulence, and bacilli 

 from different animals vary still more. Lartigau,* in an instructive 

 paper upon "Variation in Virulence of the Bacillus Tuberculosis in 

 Man," found much variation among bacilli secured from the lesions 

 of human tuberculosis. The virulence was tested by employing 

 cultures only for inoculation, and taking of each bacillary mass 

 exactly 5 mg. by weight, suspending it in 5 cc. of an indifferent 

 fluid until the density was uniform and the microscope showed no 

 clumps, and injecting into rabbits and guinea-pigs, pairs of animals 

 being injected in the same manner, with the same material, at the 

 same time, and being subsequently kept under similar conditions. 

 The occurrence of tuberculosis in the inoculated animals was de- 

 cided by both macroscopic and microscopic tests. 



Lartigau found that human tubercle bacilli from different sources 

 induced varying degrees of tuberculosis in animals; that the in- 

 jection of the same culture in different amounts produces different 

 results; that the extent and rapidity of development usually cor- 



* "Journal of Medical Research," July, 1901, vol. vi. No. i; N. S., vol. i. 

 No. I, p. 156. 



