DISEASES OF THE BLOOD. 43 



practical experience of several generations shall have 

 submitted them to as thorough a test as the Jennerian 

 system of vaccination has been able to stand successfully. 

 Pasteur, Chamberland, and Eoux find that the virus trans- 

 ferred from the dog to the ape, and cultivated by propa- 

 gation through several members of the latter order, 

 becomes progressively feebler after each inoculation-. 

 After a certain period of such cultivation, if it be hypo- 

 dermically administered to dogs, guinea-pigs, or rabbits> 

 even by intracranial injection (the most deadly method), 

 death does not result, but the animal acquires an immunity 

 from hydrophobia. If, on the other hand, the poison of 

 rabies be cultivated in successive rabbits, or guinea-pigs 

 only, its potency is intensified, and after a time is so great 

 that a fatal issue invariably follows its inoculation. The 

 poison as found in the dog is intermediate in strength 

 between that of the two methods of cultivation just men- 

 tioned. Thus, by careful selection of the medium, and 

 the stage of cultivation, it is possible to accumulate a 

 store of attenuated virus, which can be relied on to com- 

 municate a modified rabies, whose inoculation shall be 

 protective against its severer forms as that of vaccinia is 

 against variola. Since these results were obtained Pas- 

 teur has shown with regard to the virus of rabies that it 

 affects the spinal cord before the medulla oblongata j it 

 has its seat in all parts of the nervous system ; may be 

 retained with all its virulence in the brain and cord for 

 several weeks by freezing or by exclusion of air. This 

 virus has not yet been found associated with any special 

 microbe, it does not seem to be attenuated by cold, and 

 in different species of animals comes to vary greatly in 

 strength and properties when, by successive inoculations, 

 the virus has attained the fixity proper to each genus. 

 Pasteur has different kinds of rabies virus of which he 

 is able to exactly prognosticate the action on special 

 species. He believes that one of these serves to secure 

 immunity to dogs virulently inoculated. He has twenty- 

 three dogs thus rendered insusceptible of rabies. The 

 hypothesis of the passage of the virus from the bite 



