DIFFUSE CASEATION. 205 



tissues it is more marked than in others. These tissues 

 are the lungs and the lymph-glands. 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 section 

 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 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 en- 

 tire caseous area is surrounded by a zone similar to that 

 around the caseous centre of the miliary tubercles. It 

 is of special importance to recognize the connection be- 

 tween this diifuse caseation of the tissue and the tubercle 

 bacilli, because until its nature was accurately deter- 

 mined the caseous pneumonia of the lungs formed the 

 chief obstacle which many had in recognizing the infec- 

 tiousness of tuberculosis. 



CAVITY-FORMATION. The production of cavities 

 which forms 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, and a 

 cavity remains. In the wall of this cavity the tuber- 

 culous changes still proceed, both as a diffuse caseation 

 and formation of miliary tubercles. The whole cavity 

 with the reactive changes in the tissues of its walls may 

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