DESCRIPTIONS OF PATHOGENIC BACTERIA. 171 



monia in man. These bacteria are often lance- 

 shaped, and are frequently grouped in pairs, 

 for which reasons the organism has been given 

 the name Diplococcus lanceolatus. They are 

 arthrobacteria and stain by Gram's method. 

 They are so sensitive to drying that a few 

 days' desiccation is sufficient to render them 

 powerless. When shut off from access of air, 

 as in egg-cultures, they retain their virulence 

 for many weeks and even for months, and their 

 vitality for many months (Bunzl-Federn). 

 They cause a simple typical septicaemia in 

 rabbits and, when virulent, in white mice and 

 guinea-pigs also, but for the latter animals 

 large quantities are usually necessary. Dogs 

 succumb to injection with the toxin but not to in- 

 fection. In man the pneumococci induce typi- 

 cal fibrous pneumonia, which at times passes 

 over into septicaemia ; they are found besides 

 in inflammations of the serous membranes, of 

 the pleura, of the cerebral membranes, and in 

 inflammations of the middle ear. They are 

 found also living as oeco-parasites in healthy 

 persons, so that " catching cold " often affords 

 them an opportunity of invading the body. In 

 this fact lies the explanation of those familiar 

 instances in which a healthy person suddenly 

 develops pneumonia as a consequence of tak- 

 ing cold. 



