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, 
