408 APPLICATION OF METHODS OF BACTERIOLOGY 



the miliary tubercle. In certain tissues, notably the lungs 

 and lymphatics, it is more marked than in others. 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 

 opaque and of a whitish-yellow color, and on section 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 casea- 

 tion of the tissues are the simultaneous deposition of masses 

 of tubercle bacilli in them, and the involvement of a wide 

 area instead 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 sur- 

 rounded by a granulation-zone similar to that around the 

 caseous centre of the miliary tubercles. It is of special 

 importance 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 infectiousness 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 tubercles. The material softens, 

 is expelled by way of the bronchi, and a cavity results. 

 In the wall of this cavity the tuberculous changes still pro- 

 ceed, both as diffuse caseation and formation of miliary 



