516 THE RESPIRATORY SYSTEM. 



In all these various processes new-formed fibrous tissue, not tubercu- 

 lous in character, may develop, variously distorting the lungs and 

 sometimes enclosing the tuberculous areas. There is much reason for 

 the belief that the characteristic so-called tubercle-tissue formation in all 

 phases of tuberculosis is a response of the living cells to injury, which as 

 in other phases of inflammation is fundamentally conservative (see page 

 126). But whether this be so or not, it is certain that it is through the 

 development of fibrous tissue, which is so important a feature in persis- 

 tent phases of pulmonary tuberculosis, that the delimitation or replace- 

 ment of tuberculous foci, the encapsulation of tuberculous cavities, etc. , 

 occurs. While, therefore, fibrous-tissue formation in the lung is fre- 

 quently associated with the necrotic and other destructive tuberculous 

 lesions, and is often a very conspicuous factor, it should be remembered 

 that the healing which takes place in the majority of cases in man after 

 moderate infection is achieved through its agency. 



It is especially upon the presence or absence of fibrous tissue in the 

 lesions that the distinction between acute and chronic phthisis is based. 



It will be seen from this brief outline of various prominent phases 

 of pulmonary tuberculosis that the gross appearances of the lungs are 

 most diverse, although the processes by which these changes are induced 

 are few and comparatively simple. While no two lungs are quite simi- 

 lar in the complex phases of the lesion, systematic gross and micro- 

 scopical examinations soon enable the student to recognize the type of 

 lesion under great complexity of detail. 



THE DISTRIBUTION OF THE LESIONS IN PULMONARY TUBERCULOSIS. 



Aside from general miliary tuberculosis in which the tubercles are 

 widely distributed throughout one or both lungs, the most common seat 

 and starting-point of tuberculous lesions in adults is the apical region or 

 the depth of the lung, particularly the right, somewhat below the apex. 1 

 In children, the tuberculous process more frequently commences in the 

 bronchial lymph-nodes (Plates III. and XI.). 



From the apex the tuberculous process may extend downward, and 

 with various forms of lesions involve more or less of the lungs. It is 

 common to find at autopsies older fibrous lesions about the apices while 

 marks of more active processes are to be seen below (Plate XII.), In a 

 considerable percentage of bodies examined at autopsies, small and often 

 healed tuberculous foci are found at the apex or in the bronchial lymph- 

 nodes without evidence of extension of the process. 



CONCURRENT INFECTION IN PULMONARY TUBERCULOSIS. 



While it has been definitely established that tubercle bacilli are the 

 excitants of both the productive and exudative forms of tuberculous 

 inflammation, these bacilli are not infrequently associated in pulmonary 



1 For a suggestive consideration of apical vulnerability see Hutchimon, "Studies in 

 Human and Comparative Pathology," 1901, p. 81. 



