Pathogenesis 669 



Temperature Sensitivity. -The bacillus is sensitive to tempera- 

 ture variations, not growing below 29C. or above 42C. Rosenau* 

 found that an exposure to 6oC. for twenty minutes destroys the 

 infectiousness of the tubercle bacillus for guinea-pigs. 



Effect of Light. It does not develop well in the light, and when 

 its virulence is to be maintained should always be kept in the dark. 

 Sunlight kills it in from a few minutes to several hours, according to 

 the thickness of the mass of bacilli exposed to its influence. 



Pathogenesis. -Channels of Infection. The channels by which 

 the tubercle bacillus enters the body are numerous. A few cases 

 are on record where the micro-organisms have passed through the 

 placenta, a tuberculous mother infecting her unborn child. It is not 

 impossible that the passage of bacilli through the placenta in this 



Fig. 276. Bacillus tuberculosis: a, Source, human; b, source, bovine. Ma- 

 ture colonies on glycerin-agar. Actual size (Swithinbank and Newman.) 



manner causes the rapid development of tuberculosis after birth, 

 the disease having remained latent during fetal life, for Birch- 

 Hirschfeld has shown that fragments of a fetus, itself showing no 

 tuberculous lesions, but coming from a tuberculous woman, caused 

 fatal tuberculosis in guinea-pigs into which they were inoculated. 



The most frequent channel of infection is the respiratory tract, 

 into which the finely pulverized pulmonary discharges of consump- 

 tives and the dusts of infected rooms and streets enter. Fliigge, 

 Laschtschenko, Heyman-Sticher, and Benindef found that the 

 greatest danger of infection was from the atomized secretions, dis- 

 charged during cough, from the tuberculous respiratory apparatus. 

 Nearly every one discharges finely pulverized secretions during 

 coughing and sneezing, as can easily be determined by holding a 

 mirror before the face at the time. Even though discharged by con- 

 sumptives, these atoms of moisture are not infectious except when 

 there are open lesions in the lungs, etc. Experiment showed 

 that they usually do not pass farther than 0.5 meter from the patient, 

 though occasionally they may be driven 1.5 meters. A knowledge 



* "Hygienic Laboratory," Bulletin No. 24, Jan., 1908. 

 t"Zeitschrift fur Hygiene," etc., Bd. xxx, pp. 107, 125, 



139, 163, 193. 



