190 BIOLOGY OF PNEUMOCOCCUS 



ables responsible for the resistance of the individual mouse to pneu- 

 mococcal infection by studying the capacity of animals of a sin- 

 gle strain to utilize the protective properties of antipneumococcic 

 serum. The authors concluded that the important variables were 

 body-weight and the number of cells in the peritoneal cavity fol- 

 lowing the intraperitoneal injection of pneumococci and homolo- 

 gous immune serum, which in turn resolved itself into the number 

 of monocytes present. The dominant factor in determining sus- 

 ceptibility, other factors being equal, was the relation between the 

 number of monocytes and the number of pneumococci in the peri- 

 toneal cavity at the time of the injection of culture and serum. 



The resistance of mice to infection by inhalation may be de- 

 creased by the previous administration of alcohol. Stillman and 

 Branch (1924, 1930, 1931 ) 1337 ' 13401 found that inspired pneumo- 

 cocci rapidly disappeared from the lung of normal mice and rarely 

 caused septicemia, but in alcoholized mice the organisms persisted 

 in the lung for a longer period and fatal septicemia was frequent, 

 while pulmonary localization of the infection occurred in mice pre- 

 viously immunized either actively with heat-killed pneumococci or 

 passively with homologous immune serum. The observations of 

 Branch and Stillman 148 were confirmed by Lange and Keschis- 

 chian, 785 who succeeded, but only with difficulty, in inducing pul- 

 monary infection in mice through inhalation. The latter authors 

 also found mice to be resistant to percutaneous and peroral inocu- 

 lation with pneumococci. 



The frequency with which colonies of white mice are infected 

 with the so-called mouse typhoid due to Bacillus typhi murium is a 

 factor to be considered when selecting mice for experimental pur- 

 poses. The disease, so often remaining latent, may not be evident, 

 but it may nevertheless greatly alter the reaction of the animals to 

 experimental inoculation. 



THE RAT 



The rat is a near zoological relation of the mouse and is highly 



