514 THE RESPIRATORY SYSTEM. 



and necrotic portions of the lung may take place with the formation of 

 large ragged cavities (see Plate X.). In many cases enormous numbers 

 of tubercle bacilli are present in the exudate of the involved region in 

 this type of exudative and necrotic pulmonary tuberculosis. 



2. CHRONIC TYPE. There may be a more gradual development of 

 the exudate, involving larger or smaller areas, often with tuberculous 

 thickening and necrosis of the walls of the air spaces. With this as with 

 other forms of pulmonary tuberculosis there may be associated a growth 

 of simple fibrous tissue with the formation of cavities. 



Not infrequently the exudate in such forms of diffuse tuberculous in- 

 flammation of the lung is less cellular and more serous or sero-nbrinous 

 in character when the appearance of the consolidated region is trans- 

 lucent and gelatinous. 



Obliteration of the larger blood-vessels by thrombosis or inflamma- 

 tion of the wall often plays an important part in most forms of tubercu- 

 lous lesions of the lungs. When large trunks are involved, large lung 

 areas, supplied by the occluded vessels, particularly those in which exu- 

 date is present, may become necrotic en masse, then often appearing on 

 section smooth, shining, grayish-white in color, and bloodless. 



THE FORMATION OF CAVITIES IN PULMONARY TUBERCULOSIS. 



We have seen that in tuberculous broncho-pneumonia ragged cavities 

 may form by a progressive necrosis and disintegration of the thickened 

 walls of the affected bronchi and the adjacent lung tissue (Plate IX. 

 and Fig. 293). Similar destructive alterations may occur in the areas of 

 tuberculous and necrotic tissue which involve larger and smaller portions, 

 of whole lobes (Plate XL). This is most often rapid and extensive in 

 solidified and necrotic areas of the exudative type (Plate X. and Fig. 

 295). These ragged cavities may communicate with one another as well 

 as with the bronchi. 



While at first without distinct limiting walls, if the necrotic process 

 be not too active and extensive, new fibrous tissue may gradually form 

 about tuberculous cavities (Plate XII. and Fig. 296) ; and these may 

 become lined with granulation tissue or a layer of new-formed tubercle 

 tissue. Here an enormous proliferation of tubercle bacilli may occur for 

 long periods. These may be cast out in the sputum or aspirated into other 

 parts of the lungs. The old blood-vessels of the involved portion of the 

 lung may lie upon the walls or stretch across these cavities, sometimes with 

 obliterated lumina, sometimes still permeable, and from these haemorrhages 

 may occur. There may be continuous and prolonged suppuration of the 

 walls of the cavities and putrefactive processes may be incited by the 

 advent through the air passages of various forms of bacteria. The walls 

 of cavities may become fibrous with an arrest of the tuberculous process, 

 and they may become shut off from bronchial communication. 



