THE RESPIRATORY SYSTEM. 507 



in connection with various other phases of acute and chronic pulmonary 

 tuberculosis. 



When miliary tubercles are situated in the parenchyma of the lung 

 the walls of the air spaces may be visible in the new growth (Fig. 286) 

 or they may be largely or wholly obliterated (Fig. 287). 



In the obliteration of the walls of the air vesicles, their former situa- 

 tion may be indicated by structureless bands or streaks of necrotic ma- 

 terial, or they disappear altogether. 



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" iiBtS^ 



: ,^' y: 



FIG. 287.^A MILIARY TUBERCLE OF THE LUNG. 



This tubercle is largely caseous, the walls of the involved air vesicles being merely indicated in the dead 

 mass by slightly stained streaks or bands. There is cellular exudate in the surrounding air vesicles. 



Chronic and " Healed " Miliary Tubercles. The small foci of tubercu- 

 lous inflammation which are called miliary tubercles may, as we shall 

 see later, extend and coalesce so that with more or less exudative pneu- 

 monia large areas of the lung may become consolidated, thus developing 

 one of the forms of pulmonary tuberculosis called phthisis. 



On the other hand, small tuberculous foci in the lungs may with or 

 without extensive necrosis become surrounded by or converted into 

 masses of dense fibrous tissue. These fibrous tissue masses, which are 

 often called "healed tubercles" (Figs. 288 and 289), may contain necrotic 

 material, are often mottled black from aiithracotic pigment, or they may 

 be calcified at the centre. Tubercle bacilli may be absent from them or 

 the bacilli may remain alive within them for a long time quiescent, but, 

 as Lartigau and others have shown, still virulent. When such circum- 

 scribed masses of more or less fibrous tubercle tissue are scattered 

 through the lungs, although the individual masses may be of considerable 

 size, the process is sometimes called "chronic miliary tuberculosis." 



It should be remembered that the new fibrous tissue which forms in 

 and about miliary tubercles, as well as other tuberculous lesions, is the 

 result of a distinctly reparative process in which the already formed tu- 

 bercle tissue or its products apparently act somewhat as foreign bodies 

 may in inducing fibrous-tissue growth. 



