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 
