THE RESPIRATORY SYSTEM. 



505 



pulmonary pleura. They may be scattered singly through the lungs 

 (Plate IV.), or aggregated in groups (Plates VI. and VII.). They may 

 be separated by considerable interspaces, or so close together that the 

 lung is rendered nearly solid. Some are so small and transparent that 

 they can hardly be seen with the naked eye ; others are larger and more 

 opaque and may have a lighter centre marking the area of necrosis. 



In many cases it is evident that the lungs are infected through the 

 blood-vessels or the lymphatics, for the general tuberculous infection 

 is secondary to a localized tuberculosis either in the lungs or in some 



FIG. 285. A MILIARY TUBERCLK OF THE LUNG. 



Involving only two air vesicles, of which the walls are infiltrated and the cavities filled with tubercle 

 tissue. The blood-vessels of the air vesicles are injected, except where these are obliterated by the tubercu- 

 lous involvement of their walls. 



other part of the body. In a considerable proportion of cases of miliary 

 tuberculosis of the lungs tuberculous lesions of the bronchial lymph- 

 nodes or of the lung tissue at the apex of much longer standing indicate 

 the probable immediate source of origin of the widely distributed tubercle 

 bacilli. Very often in miliary tuberculosis of the lungs the tubercles in 

 one part of the lungs are larger and appear to be older than those in 

 other parts. Thus it is not infrequent to find the tubercles in the upper 

 portion of the lungs more abundant, larger, and more fibrous than in the 



