GA VITY-FORMA TION. 253 



area of tissue undergoes the same process of necrosis 

 and caseation as the centre of the miliary tubercle. In 

 some tissues it is more marked than in others. These 

 tissues are the lungs and the lymph-glands. In rab- 

 bits, particularly, all the changes in the lung frequently 

 come under this head. When this is the case solid 

 masses are found, sometimes as large as a pea, or in- 

 volving even an entire lobe or the whole lung in some 

 cases. They are of a whitish-yellow, opaque color, and 

 on section are peculiarly dry and hard. Entire lym- 

 phatic glands may be changed in this way. The con- 

 ditions for this caseation of the tissues are probably 

 given when a large number of tubercle bacillus enter the 

 tissue simultaneously and a wide area is involved, in- 

 stead of the small centre of the miliary tubercle. Ne- 

 crosis is so rapid that time is not given for those reactive 

 changes to take place in the tissues which result in the 

 formation of the outer zone of the miliary tubercle. 

 In other instances the entire caseous area is surrounded 

 by a granulation zone similar to that around the caseous 

 centre of the miliary tubercles. It is of special impor- 

 tance to recognize the connection between this diffuse 

 caseation of the tissue and the tubercle bacillus because 

 until its nature was accurately determined the caseous 

 pneumonia of the lungs formed the chief obstacle which 

 many had in recognizing the infectiousness of tubercu- 

 losis. 



Cavity-formation. — The production of cavities 

 which forms such a prominent feature in human tuber- 

 culosis, particularly in the lungs, is due to the soften- 

 ing of the necrotic caseous masses or of aggregations of 

 miliary tubercles. The material softens and is expelled, 

 and a cavity remains. In the wall of this cavity 



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