INTRODUCTORY AND GENERAL 15 



Further, the same culture can be made to undergo variation, 

 its virulence being either exalted or diminished, and this is a 

 subject of the utmost importance. An increase in virulence is the 

 more difficult to secure, and can practically only be procured by 

 passage through animals, or by other closely allied process. 



Passage is carried out thus : the avirulent culture is made to 

 infect animals either by the administration of massive doses, or 

 by the simultaneous injection of some substance which lowers the 

 local or general resistance (lactic acid, alcohol, the toxins of 

 B. prodigiosus, etc.). In any case, the organism is made to cause 

 an infection which may or may not be allowed to progress to a 

 fatal issue. From the animal thus infected a second culture is 

 made, and the material used to inoculate a second animal, and 

 the organism will be found to have undergone a noticeable access 

 of virulence. The process is repeated as often as is necessary, 

 and ultimately the virulence of the culture will be brought to 

 its highest possible pitch. The simplest method, where available, 

 is to give the injections into the peritoneum, and to make the 

 cultures by withdrawing some of the peritoneal fluid in a sterile 

 pipette, and incubating it as it is, or after the addition of broth. 



This method was introduced by Pasteur, and is of especial 

 value in preparing the vaccine used against rabies. The organism 

 of this disease is unknown, but the virus occurs in the brain, and 

 emulsions of this substance are used for inoculation. It is found 

 that the virus occurring naturally in rabid dogs (the " virus of 

 the streets ") is comparatively avirulent to rabbits. This is 

 shown by the long incubation period fifteen to eighteen days 

 after intracerebral injection. After about fifty passages through 

 rabbits, the virus becomes so exalted that the incubation period 

 is shortened to six days, and the process cannot be carried further. 

 This virus is called the " fixed virus," and its potency is main- 

 tained unaltered, no matter how many more passages are made. 



Passage does not necessarily raise the virulence of the culture 

 to all animals ; it may do so only for the species used for the pro- 

 cess, the action on other species remaining unaltered or even 

 falling. Nor is passage necessarily followed by an increased 

 degree of virulence the virus of rabies diminishes in this respect 

 when passed through apes. 



Phenomena suggesting a process akin to passage occur under 

 natural conditions. Pneumococci are frequently found in the 

 mouths of healthy persons, and are, as a rule, of feeble virulence, 



