r , 



PATHOGENIC BACTERIA. 253 



subcutaneously or into the peritoneum, or by intravenous 

 injection when liquid cultures are used. Its virulence is 

 very variable. In the sputum of a case of lobar pneumonia, 

 early in the disease, it is likely to be virulent. The virulence 

 is best maintained by repeated inoculations into mice or 



rabbits. 



FIG. 62. 



Pneumococcus showing capsule, from pleuritic fluid of infected rabbit, 

 stained by second method of Hiss. (X 1000.) 



This organism is detected very frequently in the human 

 mouth. When taken from the mouth it is not, however, 

 pathogenic to animals in many instances, being found 

 virulent in only from 15 to 20 per cent, of human mouths. 

 It is the specific cause of croupous or lobar pneumonia in 

 man. In that disease the characteristic lesion consists of an 

 inflammation of the lung, involving large areas, usually one 

 or several lobes. An exudate is poured into the air-vesicles, 

 which in the early part of the disease contains red blood- 

 cells, imparting the rusty color to the sputum. The principal 

 element in the exudate is fibrin. The formation of fibrin 

 produces the liver-like consolidation or " hepatization." 



