HYDROPHOBIA. 323 



Resistance of the Virus of Rabies (Medulla Oblongata 

 of Dogs Dead of Rabies). The virus is destroyed by ex- 

 posure for an hour to a temperature of 50 C. (122 F.) ; 

 further, to 5 per cent, carbolic acid for fifty minutes ; to 

 mercuric chlorid, I : 1000 ; to acetic acid ; to potas- 

 sium permanganate. The spinal cord of (Paris) rabbits 

 dead of rabies, kept in dry air and protected from putre- 

 faction, loses its toxicity only after fourteen or fifteen days. 

 The smaller the animal, the thinner the spinal cord, the 

 more rapidly does the loss in virulence take place. 



Immunization and Vaccination. Pasteur showed that 

 the virus of canine rabies slowly diminishes in intensity 

 when inoculated from dogs upon monkeys in a progressive 

 series. This gradual loss of virulence is distinctly appreci- 

 able in the increase in the period of incubation. If the 

 infecting material is reconveyed from monkeys to rabbits, 

 an increase in virulence takes place, which constantly aug- 

 ments on further inoculation into rabbits. The period of 

 incubation becomes shorter ; and, finally, after the hun- 

 dredth passage through the body of the rabbit, the period 

 is not longer than seven days. It was impossible to pro- 

 duce a more active virus. The virus retained its virulence 

 now unchanged, and Pasteur, therefore, designated it virus 

 fixe. Pasteur in this way prepared a series of rabies-viruses 

 that, beginning with the spinal cord of monkeys and 

 progressing to the spinal cord of rabbits that succumbed to 

 the virus fixe, possessed a steadily increasing virulence. 

 On inoculating successively dogs subcutaneously with these 

 spinal cords, of from the lowest to the highest degree of viru- 

 lence, the animals treated failed to develop rabies, but be- 

 came immune even to subdural infection with the virus fixe 

 and to the bites of other dogs suffering from ordinary 

 rabies. 



A year later, in 1885, Pasteur and his collaborators, 

 Chamberland and Roux, developed a still more practical 

 method of immunization. Proceeding from the fact already 

 mentioned that the medulla of animals suffering from rabies 

 is completely deprived of its virulence in from fourteen to 

 fifteen days by desiccation, they dried the spinal cords of 

 rabbits that had succumbed to the virus fixe for from one 

 to fourteen days with all aseptic precautions in high ster- 

 ilized glass cylinders. The spinal cord fourteen days old 

 had lost all its virulence ; that thirteen days old and that 



