IMMUNITY. 173 



containing the bacilli, is squeezed from it and heated to a 

 temperature of 95° to 99° C. for six hours. The dried material 

 mixed with water constitutes the vaccine. The Department 

 of Agriculture of the United States now furnishes this vaccine 

 free to farmers. The results of this method are said to be 

 very gratifying.* In the human disease, bubonic plague, a 

 nearly similar procedure has been proposed by Haffkine. To 

 protect against plague, cultures of plague bacilli are used 

 which have been previously sterihzed by heat, with carbolic acid 

 added as a preservative. (See section on Bubonic Plague, 

 Part IV.) 



Inoculation Against Rabies or Hydrophobia.f — ^The 

 immunity produced in this case probably depends upon prin- 

 ciples similar to those underlying the examples related on 

 the preceding pages. But this question cannot be regarded 

 as settled until the organism of rabies has been isolated and 

 cultivated. Attempts to discover this organism have, as yet, 

 been futile, though certain minute bodies, bodies of Negri, 

 have been observed within ganglion-cells of the central nervous 

 system from cases of rabies, and if has been claimed that they 

 are protozoa and the cause of the disease. Whether this is 

 true or not, Negri bodies make a most valuable means of rapid 

 diagnosis. 



Pasteur discovered that rabies could be produced in ani- 

 mals by inoculation under the dura mater with portions 

 of the spinal cord of a dog suffering from hydrophobia. He 

 also found that successive passages through a series of rabbits 

 greatly increase the virulence of the virus, as indicated by 

 a much shorter period of incubation after inoculation. The 

 first rabbit of the series inoculated with the "street" rabies 



* See Recent Annual Reports, Bureau of Animal Industry, U. S. Department 

 of Agriculture. 



■f For a review of recent works on rabies see Remlinger. BuUetin de I'Institut 

 Pasteur. II., Nos. 19 and 20. 1904. 



