THE RESPIRATORY SYSTEM. 503 



considerations and is not to be regarded as indicating fundamental differ- 

 ences in the reaction of the tissues. 



Tubercle bacilli may enter the lungs either through the blood- and 

 lymph-vessels, being brought from a focus of tuberculous inflammation 

 in another part of the body ; or, as is more frequently the case, they are 

 introduced through the air passages by the inhalation of floating dust 

 particles, among which are living tubercle bacilli. There is evidence 

 that tubercle bacilli may enter the body through the gastro-intestinal 

 mucous membrane without the development of a tuberculous lesion at 

 the point of entry. ' 



The introduction of tubercle bacilli into the lungs may induce an 

 exudative inflammation with the accumulation of fibrin, leucocytes, and 

 exfoliated epithelium in the air spaces ; a productive inflammation with 

 the growth of epithelial cells, or of round-celled tissue, or of a tissue 

 composed of basement substance, large and small cells, and giant cells 

 called tubercle tissue (see page 242) ; or there may be added necrosis of 

 the new tissue and of portions of the lung. All of these phases of tuber- 

 culous lesions may and usually do occur together. 



The character of the inflammation in each case seems to be governed 

 by the type of cells especially involved, by the number, virulence, and 

 proliferative capacity of bacilli which are introduced into the lungs, and 

 the way in which these enter, as well as by the susceptibility of the indi- 

 vidual. If a large number of virulent tubercle bacilli be inhaled or 

 aspirated through the bronchi, or if the bacilli grow with great rapidity, 

 both productive and exudative inflammations may be set up in a consid- 

 erable portion of the lungs. If, on the other hand, but few bacilli enter 

 and their proliferation or virulence be not extreme, or if these find their 

 way in small numbers into the lungs through the blood-vessels or lym- 

 phatics, then there may be small foci of productive inflammation with 

 but little exudation. The tuberculous alterations in the lungs are usually 

 accompanied or followed by a series of secondary processes which often 

 complicate the condition of the patient as well as the morphological ap- 

 pearances of the organs. The obliteration or destruction of the smaller 

 blood-vessels of the lungs in the tuberculous areas contributes to the 

 gray or whitish appearance of the lesions, due largely to new-formed tis- 

 sue or accumulated exudate. The formation of fibrous tissue in the 

 attempts at repair of the damage wrought by the tubercle bacillus often 

 dominates the structural picture. 



The traditional distinctions between acute and chronic forms of pul- 

 monary tuberculosis are often morphologically not at all well defined and 

 are of value chiefly for clinical purposes. 



It has been customary to set apart those forms of acute tuberculosis 

 of the lungs in which the lesions are in the form of small discrete so- 

 called " miliary " foci, calling the condition Acute Miliary Tuberculosis. 

 The other tuberculous lesions involving in varying degree the lungs and 

 the bronchi with associated and often extensive exudative necrotic and 

 reparative processes have been commonly jumbled together as Acute and 



1 For a discussion of the source of the tubercle bacillus in phthisis see Verh. d. 

 deutschen Path. Qes., iv., September, 1901, p. 73; also references on page 252. 



