TUBERCLE BACILLUS 217 



The disease is widespread, but is much more common where 

 human beings are huddled together in dark, badly ventilated 

 rooms and shops. In tissues, the characteristic lesion is a tuber- 

 cle. This is a globular mass, about the size of a very small shot, 

 and grayish pearly white. Microscopically, in the centre of the 

 tubercle, are found several large multinuclear cells, called giant 

 cells, which often contain thirty or more nuclei, and a number of 

 tubercle bacilli, the nuclei often being situated at one pole, while 



FIG. 66. Tubercle bacilli showing involution forms. (Kolle and 

 Wassermann.) 



the bacilli are at the other. About the giant cells epithelioid 

 cells are grouped, and about these small round cells are massed in 

 great numbers. No new blood-vessel formation is ever found 

 in the epithelial cell layers, or among the giant cells. Owing 

 to insufficient blood-supply the centre of the tubercle frequently 

 undergoes caseous degeneration. If the lesion heals, the caseous, 

 centres become calcareous, and the periphery changes into 

 connective tissue. If the tubercles coalesce, great masses of 

 caseous tissue form. If the latter becomes infected with other 



