INFi:CTIVI-: CRANULOMATA. 349 



nie bacteria multiply the quantity of their products is increased, 

 and these stimulate cellular multiplication and accumulation, and 

 thus the tubercle grows. The formation of a tubercle constitutes 

 a tissue reaction, but there is no vascularization; that is, no 

 new blood vessels are formed, and the existing capillaries in the 

 invaded tissues are finally obliterated. A tubercle is, therefore, 

 strictly non-vascular, although in the very beginning the afifected 

 zone may be hyperemic. Cells constituting a tubercle obtain 

 nutriment from adjacent tissues by absorption. Tul)ercles grow 

 b}' multiplication of the peripheral cells, the central cells becom- 

 ing degenerated after the^• have consumed all availaljle nutrition. 

 The structure and appearance of a tubercle varies according to 

 its age, thus: a tubercle in the very early stages is a cellular mass, 

 a little later the central portion of the cellulai- mass becomes 

 necrotic, and at about the same time a median zone, consist- 

 ing of bacteria, endothelial, and, in some cases, giant cells, 

 becomes evident ; the outer zone is the active zone and is com- 

 posed of bacteria, connective tissue cells and leucoc}-tes. As 

 the tubercle becomes larger the necrotic zone extends to the 

 median and outer zones to\^ard the periphery. Necrosis is 

 usually evident in tubercles that have attained the size of a pea. 

 The central necrosis is primarily of the coagulation type, but 

 the coagulated necrotic tissue may become licpiefied, always be- 

 comes caseous and usually calcified according to the quantity of 

 fluid contained. The calcification may be limited in extent, the 

 necrotic tissue containing small calcareous particles that cause 

 the necrotic tissue to have a "gritty feel," or it may be so ex- 

 tensive that the tubercle cannot he dissociated except by the use 

 of a sledge. Liquefied tubercular necrotic tissue (pus) is yellow- 

 ish in color in the ox. dirty white in hogs and yellowish in fowls. 

 It is not sticky, although it becomes quite thick and is finally 

 caseated.. 



Tubercles may varv in size from a microscopic point to large 

 masses. All tubercles'are small in the beginning and are usually 

 entirely cellular. Small cellular tubercles in which there is no 

 necrosis are designated miliary tubercles. Miliary tubercles ap- 

 pear as minute, grayish, translucent, pearl-like specks or nodules. 

 If all the lesions in an afi'ected animal are miliary in character, 

 the disease is termed miliary tuberculosis. Miliary tuberculosis 

 is most common in hogs. 



The appearance of a tubercle changes when central necrosis 

 begins. The color of caseous and calcareous tubercles varies 

 fro'm a dirtv white to a vellow color. The tubercles may or may 

 not be encapsulated. The capsule of a tubercular lesion is rela- 



