270 BACTERIOLOGY. 



the formation of miliary tubercles. In this a large 

 area of tissue undergoes the same process of necrosis 

 and caseatiou 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 bacilli enter the 

 tissue simultaneously and a wide area is involved, 

 instead of the small centre of the miliary tubercle. 

 Necrosis 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 and the tubercle bacillus, because until its 

 nature was accurately determined the caseous pneu- 

 monia of the lungs formed the chief obstacle which 

 many encountered in recognizing the specific infec- 

 tiousness of tuberculosis. 



CAVITY-FORMATION. The production of cavities 

 which form such a prominent feature in human tuber- 

 culosis, particularly in the lungs, is due to the softening 

 of the necrotic, caseous masses or of aggregations of 

 miliary tubercles. The material softens and is expelled, 



