THE RESPIRATORY SYSTEM. 



501 



a large part of a lobe is converted into dense fibrous tissue ; the pleura 

 may be thickened; there may be chronic bronchitis and bronchiectasia. 

 If interstitial pneumonia be associated with thickening of the pleura, 

 bands of connective tissue extend from the pleura into the lung, the 

 bronchi are inflamed and often dilated. When associated with chronic 

 bronchitis there are fibrous noduies around the bronchi, with more or 

 less diffuse connective tissue. 



The changes in interstitial pneumonia may occur by a slow hyperpla- 

 sia of the fibrous tissue or by the formation of granulation tissue which 



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FIG. 382. CHRONIC INTERSTITIAL PNEUMONIA. REVERSION OF EPITHELIUM IN ISOLATED AIR VESICLES. 



gradually becomes denser with contraction. Although this process is 

 primary in the interstitial tissue, exudates are often present in the air 

 spaces ; atelectasis may occur, while emphysema and bronchiectasia are 

 common. 



Sometimes a noteworthy change occurs in the epithelium lining air 

 vesicles which have been cut off from their neighbors by the new fibrous 

 tissue. The epithelial cells increase in number, become thicker, and 

 finally the small or distorted cavities may be lined with a complete in- 

 vestment of cuboidal cells (Fig. 282). This reversion of the epithelium 

 of the air vesicles to a less differentiated type occurs in many chronic 

 processes in the lungs. 



Pigmentation of the Lung. The inhalation of dust and smoke is so 

 continuous among those who live much indoors that the lungs of nearly 



FIG. 283.-ANTHRACOTIC PIGMENTATION OF THE LUNG. 



Showing pigment beneath the thickened pleura and in the thickened septa of the emphysematous lung. 



all persons from the earliest years are more or less pigmeuted. The for- 

 eign particles which get into the deeper recesses of the lungs are in part 



