CA VITY-FORMA TION. 293 



both in the human and experimental forms, than does 

 the formation of miliary tubercles. In this a large 

 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 lymph-glands. In rabbits, 

 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 involving 

 even an entire lobe or the whole lung in some cases. 

 They are of a whitish-yellow, opaque color, and on sec- 

 tion are peculiarly dry and hard. Entire lymphatic 

 glands may be changed in this way. The conditions 

 for this caseation of the tissues are probably given when 

 a large number of tubercle bacilli enter the tissue simul- 

 taneously 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 forma- 

 tion 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 im- 

 portance to recognize the connection between this dif- 

 fuse 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 infectious- 

 ness 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 softening 

 of the necrotic, caseous masses or of aggregations of 



