Microbic Diseases Individually Considered. 303 



incubation which becomes abbreviated to eight days 

 after twenty-five passages and to seven days after 

 fifty passages. The virus is then acclimated in the 

 rabbit, which dies regularly after seven days what- 

 ever generation the virus employed may belong to; 

 the virus of rabies thus exalted takes the name of 

 Jixed virus. It is exalted not only for the rabbit but 

 also for the dog itself, which always contracts the dis- 

 ease from intra-vascular inoculation of this virus, 

 whilst similar inoculation of natural virus is uncer- 

 tain in its results. 



In the guinea pig, which readily takes rabies by 

 intra-cranial inoculation of natural virus, virulence is 

 exalted by passage in series, until, after the eighth 

 inoculation, it becomes fixed. The incubation period 

 is then five days. Rabies in the guinea pig often 

 shows a distinct period of excitement; -the fixed virus 

 of the guinea pig is more active for the dog than the 

 natural virus. 



In the ape, passage in series attenuates the viru- 

 lence ; whilst the virus taken from the first ape kills 

 the rabbit in thirteen to sixteen days, that of the sec- 

 ond allows an incubation in the rabbit of fourteen to 

 twenty days, and that of the sixth, thirty days. In- 

 tra-venous inoculation of the virus of the sixth gen- 

 eration no more produces the disease in the dog, and 

 even intra-cranial inoculation is uncertain. The viru- 

 lence attenuated in the ape can be restored to its 

 original activity by a series of passages through 

 rabbits. 



Etiology and pathogeny. — Rabies is undoubtedly 

 caused bv a living virus. It is transmitted from one 

 animal to another by direct contagion. Mediate 



