200 DISEASES OF THE PARENCHYMA OF THE LUNG. 



secondarily, as a consequence of the bronchitis induced by the irri- 

 tant: 



1. We have seen that interstitial pneumonia is one of the compli- 

 cations of prolonged croupous or catarrhal inflammation of the lungs, 

 and that it results in induration of the latter. 



2. Simple collapse of the lung appears sometimes to give rise to an 

 inflammatory proliferation of the interstitial substance, resulting in in- 

 duration of the lung. 



3. The deposit of tubercle, and especially the softening of tubercu- 

 lous deposits, cancer of the lung, haemorrhagic infarctions, pulmonary 

 apoplexy, and pulmonary abscess, all produce interstitial pneumonia 

 with " nutritive " exudation ( Virchow). It is thus that the capsules 

 of connective tissue are found, which separate the products and residue 

 of the processes, above named, from the healthy lung. 



4. Interstitial pneumonia not unfrequently forms a complication of 

 chronic bronchitis, when it first involves the parts immediately around 

 the bronchus, but may extend thence, forming extensive solidification 

 of the lung. 



The occurrence of bronchiectasis as a result of chronic interstitial 

 pneumonia is easy of explanation. The space created in the thorax 

 by contraction of the lung must be compensated for by atmospheric 

 pressure. The thoracic wall sinks in as far as it is possible for it to 

 yield ; but, from the structure of the chest, this collapse is restricted 

 within somewhat narrow limits, so that a vacuum would form within 

 its cavity were it not that the bronchi become dilated by pressure of 

 the atmosphere. This process is usually described as if the contract- 

 ing tissue of the lung exerted a traction upon the bronchial wall, thus 

 dilating the tubes into spacious canals and extensive cavities. But 

 the extra bronchial traction, which the contracting connective tissue 

 exercises upon the bronchial wall, is not the only cause of bronchiectasis. 

 The discovery of diffuse or sacculated dilatations in the midst of tissue 

 which is simply collapsed, or which still contains air, compels us to 

 ascribe the origin of some cases of bronchiectasis to other sources. 

 Unfortunately, the condition in question is an extremely complicated 

 one ; and, in spite of the excellent work of Biermer upon the patho- 

 geny and anatomy of bronchial dilatation, its origin is, as yet, by nc 

 means satisfactorily explained. We must, therefore, content ourselves 

 by briefly stating that, in some cases, probably the calibre of the bron- 

 chus is enlarged by the pressure of stagnant secretion upon its inner 

 surface, especially when the resilience of the bronchial wall is impaired. 

 In other cases bronchiectasis, perhaps, is a result of atmospheric pres- 

 sure during the act of inspiration, in cases where portions of the lung 

 are incapable of expansion, other portions suffering abnormal compen 



