THE INFECTIOUS DISEASES. 245 



Before the discovery of the tubercle bacillus and while knowledge of 

 the lesions of tuberculosis was largely limited to their morphology, it 

 was natural that much stress should be laid upon the variety in structure 

 which the nodular growths called tubercles present, and that elaborate 

 classifications and groupings of tubercles were often deemed important. 



With an exact knowledge of the excitant of the, new growths and of 

 the varying phases of their development in the body, the morphological 

 peculiarities of tubercles are not now to be regarded as of such extreme 

 significance, since they for the most part indicate simply variations in the 

 local effect of a definite poison. These variations are due as we have 

 seen to differences in the amount and intensity of the poison, to the degree 

 of susceptibility of the individual, to the structure of the particular tissue 

 or organ involved, and to the extent and variety of local complications 

 caused by other agencies. 



It is, however, usually convenient and sometimes important to recog- 

 nize structural types in miliary tubercles. Thus they may be composed 

 wholly of small spheroidal cells "tymphoid tubercles," or of larger poly- 

 hedral cells 1 " polyhedral-celled tubercles," or of both forms of cells together 

 and with or without a new-formed stroma; or of any of these combina- 

 tions with giant cells. Then coagulation necrosis, which may occur in 

 tubercles of any type ; development of new dense connective tissue ; as- 

 sociation with various phases of simple exudative inflammation all of 

 these contribute to the variety in the structural types of miliary tuber- 

 cles. 



Diffuse Tuberculous Inflammation (Diffuse Tubercle). 1. If the infec- 

 tion with tubercle bacilli be extensive, or if step by step the bacilli are 

 distributed in the tissues about the primary seat of infection, consider- 

 able amounts of tubercle tissue of one or other form may develop and 

 pass into the condition of coagulation necrosis, so that at length large 

 necrotic masses, with a comparatively small amount of well-defined 

 tubercle tissue, either diffuse or in the form of grauula, may alone re- 

 main to indicate the character of old and slowly progressive local infec- 

 tion. This form of lesion is found in the large tuberculous masses in the 

 brain, in the mucous membrane of the bronchi, in large flat masses on the 

 serous membranes, and in the diffuse, cheesy infiltration of the lymph 

 nodes, kidneys, ureters, bladder, prostate, testicle, and uterus. 



These large areas of tuberculous inflammation are apt to be white or 

 yellow in the central and necrotic portions, which are sometimes dense, 

 compact, and hard, sometimes soft and friable. These areas are not in- 

 frequently surrounded by an irregular gray zone of tubercle tissue or by 

 a dense fibrous-tissue capsule. 



2. In marked contrast with the phase of diffuse tuberculous inflam- 

 mation just described, though often associated with it, is that in which 

 the formation of inflammatory exudates is a prominent feature. This 

 exudative form of tuberculous inflammation is best exemplified in the 



1 Some of these larger cells are regarded by many as so-called " plasma cells " (see 

 p. 103). 



