506 BACTERIOLOGY 



susceptible animals, but injections made through the 

 thoracic walls into the substance of the lung may 

 induce a typical fibrous pneumonia. This was first 

 demonstrated by Talamon, who injected the fibrinous 

 exudate of croupous pneumonia, obtained after death 

 or drawn during life from the hepatized portions of 

 the lung, into the lungs of rabbits. 



Attenuation of Virulence. The pathological changes 

 above mentioned apply only to the effects produced 

 by fully virulent cultures on susceptible animals. With 

 attenuation of virulence in the cultures or decrease 

 of susceptibility in the animals different effects are 

 produced. When the disease takes a rapid course the 

 local reaction and the changes in the internal organs 

 are comparatively slight; but the longer the process 

 lasts the greater will be the local reaction and patho- 

 logical lesions in the body. Attenuation of virulence 

 may be produced in various ways. The loss of viru- 

 lence which occurs when the micrococcus is trans- 

 planted in cultures through several generations has 

 already been referred to. A similar attenuation of 

 virulence takes place also spontaneously in the course 

 of pneumonia. Patella has shown by daily puncture 

 of the lung in different stages of the pneumonic pro- 

 cess that the virulence of the organism diminished as 

 the disease progressed, and that the nearer the crisis 

 was approached the more attenuated it became a fact 

 which has been confirmed by others. Welch found 

 that the most virulent micrococci were contained in 

 the freshly hepatized portions of the lung. Fraenkel 

 and Weichselbaum showed that the cocci taken from 

 the lung varied in virulence according to the stage of 

 the disease when they were obtained. Attenuation of 



