THE BACILLUS OF TUBERCULOSIS 295 



tuberculosis, and finally death at the end of several weeks or months. 

 Subcutaneous inoculations are less effective, and in small doses 

 frequently do not kill. Intravenous and intraperitoneal injections 

 usually produce general tuberculosis and death at the end of a few 

 weeks. 



Of other susceptible animals, field-mice and cats are readily infected 

 by artificial inoculations of tuberculous material; rats, white mice, and 

 dogs only when very large doses are given. All these animals present 

 the anatomical lesions of miliary tuberculosis. Canaries are also 

 susceptible to inoculations of the tubercle bacillus, but not sparrows. 

 Fowls and pigeons are only moderately susceptible to the bacillus 

 derived from man. Among the larger birds, parrots alone would seem 

 to be clearly susceptible. 



ANIMAL INFECTION BY NATURAL METHODS. Besides the artificial 

 modes of infection referred to, tuberculosis may be caused in animals 

 by feeding them with tuberculous material. In this case evidence of 

 infection is usually shown in the mesenteric glands, while the intestinal 

 walls are frequently not affected. Bacilli accompanied by fat much 

 more readily pass through the intestinal mucous membrane or that 

 of the tonsils and pharynx. It is certain that tuberculous infection may 

 be caused, under certain conditions, by absorption through serous or 

 mucous membranes without the evidence of any local lesion. 



The experimental production of tuberculosis by inhalation of bacilli 

 has been demonstrated by Koch in guinea-pigs, rabbits, rats, and mice, 

 and his results have since been confirmed by many others; but in these 

 experiments the bacilli were usually inhaled in the form of a very thin 

 spray in which they were suspended. The experimental inhalation of 

 dry tuberculous dust has less often proved successful. 



Various other tuberculous affections which are natural in man have 

 been produced experimentally in animals, as, for instance, tuberculosis 

 of the joints, tuberculous abscess, etc. 



ACTION UPON THE TISSUES OF THE POISONS PRODUCED BY THE 

 TUBERCLE BACILLUS. Soon after the introduction into the tissues of 

 tubercle bacilli, either living or dead, the cells surrounding them begin 

 to show that some irritant is acting upon them. The connective-tissue 

 cells become swollen and undergo mitotic division, the resultant cells 

 being distinguished by their large size and pale nuclei. A small focus 

 of proliferated epithelioid cells is thus formed about the bacilli, and 

 according to the intensity of the inflammation these cells are surrounded 

 by a larger or smaller number of the lymphoid cells. When living 

 bacilli are present and multiplying the lesions progress, the central 

 cells degenerate and die, and a cheesy mass results, which later may 

 lead to the formation of cavities. Dead bacilli, on the other hand, give 

 off sufficient poison to cause the less marked changes only, and never 

 produce cavities (Prudden and Hodenpyl). Of the gross pathological 

 lesions produced in man by the tubercle bacilli the most characteristic 

 are small nodules, called miliary tubercles. When young, and before 

 they have undergone degeneration, these tubercles are gray and trans- 



