BACILLUS OF TUBERCULOSIS. 279 



surface. Localized skin tuberculosis is sometimes pro- 

 duced by inoculation at autopsies. 



Infection by Inhalation of Tuberculous Dust. Cer- 

 tainly one of the common modes of infection is by 

 means of tuberculous sputum, which, being coughed 

 up by consumptives and carelessly expectorated, dries 

 and distributes numerous virulent bacilli in the dust. 

 As long as the sputum remains moist there is no 

 danger of dust infection, but only of direct contact; 

 it is only when it becomes dry, as on handkerchiefs, 

 bedclothes, and the floor, etc., that the dust is a 

 source of danger for infection. A great number of 

 the expectorated and dried tubercle bacilli undoubtedly 

 die, especially when exposed to the action of direct 

 sunlight; but when it is considered that from one-half 

 to three billion virulent tubercle bacilli (according to 

 the experiments of Nutall) may be expectorated by a 

 single tuberculous individual in twenty-four hours, it 

 is evident that even a much smaller proportion than are 

 known to stay alive will suffice in the immediate 

 vicinity of consumptives to produce infection unless 

 precautions are taken to prevent it. The danger 

 of infection is greatest, of course, in the close neigh- 

 borhood of tuberculous patients who expectorate pro- 

 fusely and indiscriminately that is, without taking 

 the necessary means for preventing infection. There 

 is comparatively little danger of infection at a distance, 

 as in the streets, for instance, where the tubercle bacilli, 

 even if present in the dust, have become so diluted that 

 they are not much to be feared. Exhaustive experi- 

 ments made by many observers have shown that parti- 

 cles of dust collected from the immediate neighborhood 

 of consumptives, when inoculated into guinea-pigs, 



