594 PATHOGENIC MICROORGANISMS 



tions. The lesions are fundamentally alike wherever they occur, though 

 in their detailed histological structure they may vary somewhat accorck 

 ing to the tissue in which they appear. They begin as microscopic 

 agglomerations of concentrically arranged epithelium-like, or epitheloid 

 cells, around which eventually lymphocytes accumulate. These micro- 

 scopic tubercles may gradually enlarge individually, or they may grow 

 by coalescence with neighboring tubercles. Characteristic giant cells, 

 with marginal centers of nuclei, appear near the centers of the tubercles 

 and in these giant cells the tubercle bacilli are usually found. As the 

 tubercles grow in size, the central mass becomes necrotic. Fluid pus 

 does not form, and the centers assume a grumous and friable condition 

 which is generally described as caseous or cheesy. Such tubercles may 

 result from the injection of dead bacilli, as well as living ones as Prudden 

 and Hodenpyl have shown. The cheesy degeneration may be in part 

 due to the toxic action of the substances of the tubercle bacilli and in 

 part by pressure and lack of vascularization. It is astonishing how 

 difficult it is to find tubercle bacilli histologically in such lesions. This 

 may be due perhaps to the fact that, owing to degeneration, most of 

 the tubercle bacilli have lost their acid-fastness. If tubercles heal, as 

 they often do, they undergo a fibrinous change, are surrounded by con- 

 nective tissue and, if central necrosis has begun at the time that healing 

 sets in, calcification results. For more detailed descriptions we refer 

 the reader the Text books of Pathology of Adami's, MacCallum's or 

 Delafield and Prudden. 



There has been a great deal of discussion concerning the manner in 

 which tubercle bacilli enter the human and animal body in the course of 

 spontaneous infection. A thorough discussion of this will be found in 

 the recent book by Calmette, L' Infection Bacillaire de la Tuberculose 

 (Masson, Paris, 1920), Chapter 8, p. 110. Calmette believes that when 

 a tubercle bacillus "is deposited on the surface of the skin or a mucous 

 membrane or is introduced into the healthy body by another route" it 

 becomes the prey of leucocytes which carry it into the lymphatic circu- 

 lation and into the blood. The leucocytic enzymes are not capable of 

 digesting the organism and eventually the organism is deposited in the 

 lymphatics when the leucocyte degenerates. 



Tubercle bacilli may remain latent in the body, in lymph nodes, 

 especially, for long periods. It appears that the point of entrance of the 

 tubercle bacilli into the body may be through the tonsils, and secondarily 

 thence through the lymphatics, then to other organs. Pulmonary in- 

 fection may be either by direct inhalation or indirectly through the 



