Development of the Tubercle. 531 



is caused by a specific toxin produced by tlie bacilli and results 

 in the characteristic product of the disease, the tubercle. Sur- 

 rounding the bacilli are grouped round cells (according to 

 Baumgarte'n these are derived from connective tissue cells and 

 epithelial cells, according to Borrel they are large mononuclear 

 leucocytes). These round cells become transformed into 

 epithelioid cells, with vesicular nuclei, which enclose several 

 tubercle bacilli. As a result of repeated karyokinesis which 

 process the diseased protoplasm can no longer follow (Baum- 

 garten, Weigert) or as a result of the confluence of several 

 epithelioid cells (Metchnikoff) there develop in the center of 

 the cell mass polynuclear epithelioid cells as well as true giant 

 cells. These also frequently contain tubercle bacilli. Later, 

 as a result of reaction of the surrounding tissue, mononuclear 

 lymphocytes and polynuclear leucocytes migrate from the ad- 

 jacent blood vessels, form a dense zone around this cell group 

 and lodge themselves between the epithelioid cells while a 

 fibrinous network of coagulated exudate is deposited between 

 the cells of this group. As a result of the toxic action of the 

 chemical products of the bacilli, perhaps also on account of 

 the nonvascular condition of the nodules and therefore of in- 

 sufficient nutrition (Virchow), the completed tubercle suffers 

 regressive metamorphoses. The central cells die off, break 

 down, and together with the scanty connective tissue and the 

 intermingled exudate are converted into a granular, cheesy 

 mass in which lime salts are eventually deposited. 



In cattle and swine calcified and caseated tubercles may contain 

 virulent bacilli, so that this metamorphosis does not indicate complete 

 recovery. The virulence of these bacilli is, however, as a rule, reduced 

 (Lubarsch, Schmitz). 



The structure of the tubercle in different species of animals 

 exhibits variations of minor importance, which nevertheless 

 are responsible for the often characteristic microscopical ap- 

 pearance of the lesions in different species of animals. The 

 tubercle of cattle is characterized by an abundant groundwork 

 of connectiVtC tissue frequently containing giant cells but con- 

 taining comparatively few small round cells. These tubercles 

 have a great tendency to become calcified. As a result of this 

 dense connective tissue groundwork numerous adjacent tuber- 

 cles frequently become confluent and form tumors resembling 

 fibrosarcomata in appearance. The tubercles of sheep and 

 goats have a similar structure. The tubercle of the horse is 

 well supplied with cell substance and consequently readily 

 undergoes softening. This softening process may, however, 

 not set in until at a late stage of its development. Calcification 

 is only rarely observed. The tubercle in swine is well supplied 

 with small round cells with little connective tissue ground sub- 

 stance. They readily become caseated and not infrequently 

 calcarious. The tubercle of the dog often undergoes mucoid 

 degeneration, or softening, in its later stages. 



