Tuberculosis. 293 



micromillimeters in width (about one-fourth to three-fourths the 

 diameter of a red blood corpuscle). Koch also pointed out the possi- 

 bility and methods of growing the tubercle bacillus in artificial nutri- 

 tive media outside the animal body ; and thousands of experiments 

 have contributed to the proof that by inoculation of artificially culti- 

 vated germs tuberculosis may be reproduced in all its forms in any 

 of the warm-blooded animals. 



The tubercle bacillus does not 

 grow free in nature ; occurring only v^ \ 



where it is deposited with tuberculous *"*" V 



discharges. It multiplies in this cli- 

 mate but not outside the animal body ^ — 

 and only within the organs of the "Ny " ^ 

 animal or human economy. In the '/ v, ^ 

 free state the germ of tuberculosis — --x^ 

 may retain its vitality and virulence 

 when dried and in the dark for a 

 long time (more than a vear) ; but a 



* ^ - Fig. 61. 



variety of conditions may serve to tubercle bacilli ia bronchial 

 render the infectious material harm- ^ucus of cow; x about 



less or destroy it. Sunlight, for ex- 

 ample, will kill the organism in as short a time as two to four 

 hours and even dififuse daylight will destfoy it. Rain has a favor- 

 able influence in washing away the vehicle of the contagion and 

 thus exposing it to light; a temperature of 50° -70° C. developing 

 in manure wall also kill it. 



For such reasons tuberculosis must be regarded as a true con- 

 tagious affection. Its transmission from one human being to another 

 occurs chiefly from the expectorated sputum, which, dried and blown 

 about as dust, gains access to the pharynx and air passages by in- 

 halation, sometimes a portion being swallowed and finding its way 

 from the intestines into the . tissues or sometimes infecting the 

 pharyngeal tissues and air passages. Tiny drops of saliva or 

 mucus containing the organism may sometimes be discharged in 

 talking or sneezing and in case these are directly inhaled by a 

 second individual, or if they fall upon the food and are ingested 

 with it, they may give occasion for development of tuberculosis. 

 The number of tubercle bacilli in such expectorated matter may be 

 enormous ; in a single drop of microscopic size there may be hun- 

 dreds and it has been estimated that one consumptive human being 

 may give off seventy-two hundred millions of tubercle bacilli in the 

 course of a single day (Heller). 



