442 THE TUBERCLE BACILLUS GROUP 



miliary tubercles, larger areas of caseation may develop; epithelioid 

 cells, lymphocytes, and giant cells are usually found closely packed 

 around these areas. 



The destruction of the capillaries and the resulting avascular tissue 

 helps in the necrosis of the tubercle by cutting oft 7 the blood supply. 



What is generally known as consumption or destruction of the lung 

 tissue is probably not due to the action of the tubercle bacillus alone, 

 but to secondary infection and liquefaction of tissue by other organ- 

 isms, as the streptococcus, staphylococcus, pneumococcus, or Micro- 

 coccus tetragenus. If a caseous necrotic tubercle located near a 

 bronchus ruptures into this bronchus, a large amount of tuberculous 

 material is suddenly swept into the regional areas of the lung, over- 

 whelming it and setting up a rapidly fatal infection which is known 

 as galloping consumption or phthisis florida. If a caseous tubercle 

 ruptures into a lymph or bloodvessel, the material may be carried 

 very widely through the body, causing generalized miliary tuber- 

 culosis, which resembles typhoid fever clinically. Hemorrhage not 

 infrequently takes place from the lung, due to the erosion and subse- 

 quent bursting of a bloodvessel which may have been included in the 

 caseous area. In the human lung it is practically always possible to 

 find old lesions at the apices when the infection is due to the human 

 type of the tubercle bacillus. Uncommonly no old healed tubercles 

 can be found, and the lungs are filled with miliary tubercles, in which 

 case the infection is usually caused by the bovine type of the tubercle 

 bacillus. Tubercle bacilli ingested with milk or other foods may cause 

 tubercle formation in the mesenteric glands with lesions in other parts 

 of the body. Metastatic nodules are found occasionally in the brain, 

 meninges, and epiphyses of bones. 



Pathogenesis for Lower Animals. Generally speaking, the human 

 type of the tubercle bacillus is less virulent for lower animals than the 

 bovine type. Monkeys in captivity, however, are susceptible to 

 both types, and even infection with the avian type has been found 

 in them. The course of the disease, which is spontaneous, runs similar 

 to that of human consumption, with, however, a greater tendency 

 toward generalized invasion. Goats, sheep, and horses are not as a 

 rule infected with the human tubercle bacillus. Cattle are very 

 rarely infected with the human type. Dogs and cats are said to be 

 infected occasionally. 



Rabbits. Rabbits are not as susceptible to the human tubercle 

 bacillus as the guinea-pig. Subcutaneous injections of the human 



