354 DISEASES OF THE RESPIRATORY ORGANS. 



greatly thickened even as much as a quarter of an inch at the 

 apices and fibrous, and may be studded with gray gelatinous 

 nodules. On sections, most characteristic lesion is noted : the 

 cavity, which results from liquefaction of caseous areas (Fig. 

 157). They are most frequent in the upper lobes, and vary in 

 size from that of a hazelnut to that of a small orange, or even 

 much larger, and have glistening, firm fibrous walls. Espe- 

 cially numerous in the upper part of the lungs are noted 

 irregularly rounded, opaque, grayish or yellowish areas of 

 consolidation. 



The histological changes are similar to those in acute 

 phthisis, except as modified by the longer duration of the 

 inflammatory process. In the centre of these areas the 

 tubercles cannot be distinguished on account of the caseous 

 degeneration which has taken place ; at the periphery, however, 

 they may be. Here and there are nodules which are found to 

 be composed of several histological tubercles. The centre of 

 such a system of tubercles is generally caseous, though it may 

 be firm and fibrous. These tubercles are situated in the inter- 

 alveolar and interlobular septa, and may be seen in various 

 stages of development. They are generally surrounded by 

 patches of catarrhal or fibrinous exudation. 



Fibroid Phthisis. 



In fibroid phthisis, which is the most chronic form of the 

 disease, the reparative efforts on the part of nature predomi- 

 nate over the destructive effects produced by the tubercle 

 bacilli. An area of caseation is often found to be surrounded 

 by a capsule of more or less fully developed fibrous tissue, and 

 thus its further extension is arrested. There is always an 

 especially marked increase in the amount of interalveolar and 

 interlobular connective tissue, which leads to extensive indu- 

 ration of the lung-tissue. 



Tuberculosis is often engrafted upon simple chronic inter- 

 stitial pneumonia. 



The complications of phthisis are numerous and important. 

 The pleura is nearly always implicated. In acute cases, 

 over pneumonic areas, it is usually the seat of a seropurulent 

 or fibrinous exudation, as in lobar pneumonia ; or a diffuse 



