THE PNEUMOCOCCUS 409 



but is regained on passage through a susceptible animal, 

 or by growing in fluid serum. A good deal of variation 

 occurs in the morphology of the organism obtained from 

 different sources and under cultivation. The thermal 

 death-point of the S. pneumonia according to Sternberg 

 is 52 C., the time of exposure being ten minutes, and it is 

 readily destroyed by the ordinary germicides, by light, and 

 by desiccation ; but in dried sputum it may retain its 

 vitality and virulence unimpaired for weeks. 



Pathogenic action. The S. pneumonia is pathogenic for 

 a number of animals, the most susceptible being mice, 

 then in decreasing order, rabbits, rats, guinea-pigs, and 

 dogs. Pigeons and fowls are immune. Death follows 

 after subcutaneous, intravenous, intraperitoneal, or intra- 

 thoracic injection of a virulent culture, or of rusty pneu- 

 monic sputum, into mice and rabbits in twenty-four to 

 forty-eight hours. The virulence of the organism varies 

 considerably ; under cultivation it may be completely 

 lost, while by a series of passages through a susceptible 

 animal it may be much increased. The less virulent it is 

 the longer it tends to retain its vitality under cultivation. 

 Except when injected into the lung or into the trachea, 

 pneumonia does not result, but the disease runs the course 

 of a septicaemia with high temperature and dyspnoea, 

 death being generally preceded by a subnormal temperature 

 and often convulsions. The post-mortem appearances are 

 much oedema and inflammatory infiltration at the seat of 

 inoculation, hemorrhages in the serous membranes, enlarge- 

 ment and congestion of the spleen, and congestion of the 

 lungs. The organisms occur in large numbers in the 

 blood, lungs, and spleen, usually in the form of oval 

 diplococci with well-marked capsules (Plate XVI. a), but 

 sometimes as short chains of streptococci. When injected 

 into the lung or trachea a typical fibrinous or croupous 

 pneumonia results. 



