DIPLOCOCCUS PNEUMONIA 361 



are more resistant, but still may be infected with large doses. Young 

 animals are usually more susceptible than adults. Birds are practically 

 immune. 



The results of pneumococcus inoculation into susceptible animals 

 vary according to the size of the dose, the virulence of the introduced 

 bacteria, the mode of administration, and the susceptibility of the 

 subject of the inoculation. Subcutaneous inoculation of virulent 

 pneumococci into mice and rabbits usually results in an edematous, 

 often fibrinous exudation at the point of inoculation, which, in all cases 

 n which the dose given has not been extremely small, leads to septicemia 

 and death within twenty-four to seventy-two or more hours. When 

 the dose has been extremely small or the culture unusually attenuated, 

 a localized abscess may be the only result. Intravenous inoculation is 

 usually more rapidly fatal in these animals than the subcutaneous 

 method. Intraperitoneal inoculation in rabbits results in the formation 

 of a rapidly spreading peritonitis in which the inflammatory exudate in 

 many cases exhibits differences from similar exudates produced by the 

 streptococcus. Pneumococcus exudates are apt to be thicker, to be 

 accompanied by a deposit of fibrin, and to lack the transparent red color 

 so often caused by the hemolyzing streptococci. With very virulent 

 strains, these differences are less marked. In almost aU of these infec- 

 tions death is preceded by septicemia and the microorganisms can be 

 recovered from the heart's blood of the victims. 



After such infections, the animals exhibit a rise of temperature, at 

 times visible depression, and, rarely, diarrhea. General hyperemia of 

 the organs with secondary effusions in the pleural cavities and often 

 hemorrhages upon the serous surfaces may be found at autopsy. 



The production in animals of lesions comparable to the lobar pneu- 

 monia of human subjects has been the aim of many investigators. 

 Wadsworth,^ recognizing that such lesions probably depended upon the 

 partial immunity which enabled the infected subjects to localize the 

 pneumococcus processes in the lungs after infection by way of the 

 respiratory passages, succeeded in producing typical lobar pneumonia 

 in rabbits by partially immunizing these animals and inoculating them 

 intratracheally with pneumococci of varying virulence. By this method 

 he actually carried out, for the first time, Koch's postulates in regard to 

 lobar pneumonia. 



In man, the most frequent lesion produced by the pneumococcus is 



' Wadsworth, Amer. Jour. Med. Sci., May, 1904. 



