Lesions 717 



As a rule, tubercles progressively, increase in sizie by the invasion 

 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 tuber- 

 culosis. 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 'decided 

 by both macroscopic and microscopic tests. 



. Lartigau found that human tubercle bacilli from different sources 

 induced varying degrees of tubercuolsis 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- 

 respond to the virulence of the culture; that doses of i mg. of a 

 very virulent culture may induce general tuberculosis in rabbits 

 in a very short time; that 20 mg. of a bacillus of low virulence may 

 fail to produce any lesion in rabbits or guinea-pigs; that no mor- 



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

 No. 1, p. 156. 



