320 SPECIFIC MICRO-ORGANISMS 



in 1805, by extensive post-mortem sttidies recognized the essential 

 pathological unity of tuberculous processes. Villemin, in 1865, 

 conclusively demonstrated its transmissibility by successful 

 inoculation of animals with tuberculous tissue from man and from 

 cattle. 



The response of the infected tissue to the presence of the 

 tubercle bacillus results in a localized mass of granulation tissue, 

 the tubercle, of which the histological structure is so characteristic 

 that the presence of tuberculosis may be recognized by it alone. 

 From the point of introduction the bacilli may be distributed 

 by the lymph or blood stream or may be carried by wandering 

 cells: Eventually a bacillus comes to rest and grows slowly 

 in the intercellular spaces of connective tissue. Very, soon, the 

 neighboring fixed tissue elements, connective-tissue cells and 

 endothelial cells, begin to multiply by karyokinesis and at the 

 same time the cells become swollen with nuclei large and bladder- 

 like, forming the so-called epithelioid cells. The bacilli are 

 found in and between these cells. As the pathological process 

 continues the nucleus of an occasional epithelioid cell divides 

 many times without division of the cytoplasm, giving rise to a 

 multi-nucleated giant cell. Very early in its development the 

 peripheral portion of the tubercle becomes infiltrated with lympho- 

 cytes and later, as the giant cells are formed, numerous poly- 

 nuclear leukocytes are also present. Newly formed blood vessels 

 are absent. With further extension, the center of the tubercle 

 undergoes a caseous necrosis and liquefaction, and eventually 

 this' necrotic center enlarges so as to break through an epithelial 

 surface to a passage to the exterior. This gives rise to open 

 tuberculosis and tubercle bacilli may usually be found in the 

 discharge from the lesion at this stage. 



The tubercle is the essential histological unit of tuberculosis. 

 An infiltrated tissue may contain myriads of these tubercles in 

 all stages of evolution. At any stage in its evolution the develop- 

 ment of the tubercle may become arrested and it may retrogress 

 and heal if the infected tissue is able to overcome the bacilli. 



