196 BACTERIA 



but is much more common where human beings are huddled to- 

 gether in dark, badly ventilated rooms and shops. In tissues, the 

 characteristic lesion is a tubercle. This is a globular mass, about 

 the size of a very small shot, and grayish pearly white. Microscopi- 

 cally, in the center of the tubercle, are found several large multinu- 

 clear cells, called giant cells, which often contain thirty or more nuclei, 

 and a number of tubercle bacilli, the nuclei often being situated atone 

 pole, while the bacilli are at the other. About the giant cells epithe- 

 lioid cells are grouped, and about these leucocytes (phagocytes) 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 center of the tubercle frequently 

 undergoes caseous degeneration. If the lesion heals, the caseous 

 centers become calcareous, and the periphery changes into connec- 

 tive tissue. If the tubercles coalesce, great masses of caseous tissue 

 form. If the latter becomes infected with other pathogenic bacteria 

 (streptococci and pneumococci) rapid softening occurs, with cavity 

 formation, etc. Tubercles may develop in any organ or tissue 

 of the body. The lungs, intestines, peritoneum, glands, larynx, 

 spleen, and bones become infected. The liver and pancreas seem 

 to resist invasion more than other organs. Bacilli are rarely found 

 in the blood in tubercular diseases. They may, however, be found 

 in the urine, in kidney, or bladder tuberculosis. Milk from tuber- 

 culous cows, with infected udders, often contains bacilli, and is 

 certainly a means of transmitting the disease. Cerebro-spinal 

 fluid, in tubercular meningitis, often contains the bacilli. Bacilli 

 may penetrate mucous membranes, and not cause any local lesions, 

 but infect distant organs. Tuberculosis may be spread in the 

 body in four ways. Sputum may be swallowed and infect the 

 intestines, or it may attack the larynx from the lungs. Infection 

 may spread by continuity, by the lymph stream, or by the blood. 

 Ingestion of bacilli may cause intestinal ulceration and invasion 

 of the peritoneum, also the tonsils. If the bacilli reach the blood 

 stream, the disease produced is generally acute miliary in type. 



