MORBID ANATOMY I9I 



one to ten or more millimeters in diameter or ot bunches of 

 closely set tubercles which are more or less flattened or irregu- 

 lar in shape, owing to their mutual pressure. Sometimes 

 these tubercles are attached to the serous membrane by a small, 

 tough, fibrous pedicle ; frequently, however, this is absent 

 and the nodules rest bodily upon the membrane. 



The structure of the tubercle consists in the beginning of a 

 few cells surrounding the invading specific organisms. These 

 are soon encased by a zone of epithelioid cells and giant cells 

 which is soon surrounded by an outer layer of round or 

 lymphoid cells. The central portion becomes necrosed and as 

 the nodule enlarges the central necrotic portion becomes cor- 

 respondingly large. 



This histological structure of the tubercle is typically illus- 

 trated in the beginning avian tubercle. In cattle there is a 

 strong tendency for the necrotic tissue to become infiltrated 

 with lime salts and encapsulated. In certain species a deposit 

 of fibrous tissue in the outer zone of the tubercle has been 

 observed. In the smaller and more susceptible experimental 

 animals such as the guinea pig and rabbit and frequently in 

 swine, the lesions are of a more diffuse nature infiltrating the 

 interstitial tissue with the tuberculous mass and gradually 

 encroaching upon the parenchyma. Circumscribed tubercles 

 may also be present. 



In secondary or generalized tuberculosis one or more of 

 the organs, such as the omentum, serous membranes, or lym- 

 phatic system, may become more or less thickly sprinkled with 

 minute grayish nodules about the size of a millet seed. These 

 tubercles are at first almost the color of mother-of-pearl but 

 later as the central caseous degeneration begins they become 

 grayish. Giant cells are usually numerous. 



In studying the lesions in a fatal case of tuberculosis one 

 may find with varying modifications one or more of the follow- 

 ing conditions : 



I . The primary lesion may be found in any one of the 

 organs or membranes. Its comparative age is determined by 

 the character of the anatomical changes. It may be entirely 



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