334 BACTERIOLOGY. 



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 certain tissues, notably the lungs and lymphatics, 

 it is more marked than in others. In rabbits, par- 

 ticularly, 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 

 which appear to be most favorable to the occurrence 

 of this widespread caseation of the tissues are the 

 simultaneous deposition of a large number of tubercle 

 bacilli in them, and the involvement of a wide area in- 

 stead of a single isolated point, as in the miliary tubercle. 

 Necrosis is so rapid that time does not suffice 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 im- 

 portance to recognize the etiological connection between 

 this diffuse caseation and the tubercle bacillus, because 

 until its nature was accurately determined 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, a 

 prominent feature in human tuberculosis, particularly 

 of the lungs, is due to softening of the necrotic, 

 caseous masses or of aggregations of miliary tuber- 



