534 PROTOZOA 



positions say, the cerebrospinial fluid had always the same action 

 in the same species; but that fluid taken from an animal of different 

 species was weaker or stronger as the case might be. Thus the cerebro- 

 spinal fluid of a series of dogs is of constant strength and inoculations 

 made from dog to dog regularly produce death from rabies, the animals 

 passing through an incubation period fairly constant in length, and 

 through a series of similar symptoms up to death at the expiration of the 

 same term. If, however, a series of mortkeys be inoculated the virus gradu- 

 ally becomes attenuated, and this attenuation becomes more and more 

 marked in successive inoculations until eventually, after the disease has 

 run a longer and longer course in the successive animals, there comes a 

 time at which the virus is no longer sufficiently active to cause death. 

 If this attenuated fluid be now passed through a series of rabbits, dogs, 

 or guinea-pigs it comes back to such a strength that it will kill, though 

 slowly; then, however, its virulence gradually increases until the original 

 intensity is reached. If successive inoculations be made into rabbits 

 with fluid, either from the dog or the monkey, the virulence may be so 

 exalted beyond that of the virus taken from a street dog, in which the 

 incubation period is from twelve to fourteen days, that at the end of 

 the one hundredth passage the incubation period may be reduced to 

 about six or seven days. This, the strongest virus obtainable, was 

 called by Pasteur the "fixed virus." Rabic virus appears also to 

 become attenuated under certain conditions of temperature; indeed, 

 if it be subjected for about an hour to 50 C., or in half an hour 

 if to 60 C., its activity is completely destroyed. A 5 per cent, solu- 

 tion of carbolic acid, acting for the same period, exerts a similar 

 effect, as do likewise 1 : 1000 solutions of bichloride of mercury, acetic 

 acid, or potassium permanganate. The virus also rapidly loses its 

 strength by exposure to air, especially in sunlight; when, however, pro- 

 tected from heat, light, and air it retains its virulence for a long period. 

 In his earlier experiments Pasteur selected a series of rabic poisons 

 of different strengths, beginning with that obtained from the spinal 

 cord of the monkey from the very weak to the strongest that he could 

 obtain in this animal then passing through a similar series obtained 

 during the process of exaltation of the virus by passage through the 

 rabbit. By inoculating dogs subcutaneously with virus taken from a 

 series commencing with the weakest taken from a monkey, and grad- 

 ually working up to that obtained from the rabbit from the earliest 

 to the latest in the series the animals become immune not only against 

 subcutaneous injection, but against subdural infection with fixed virus, 

 and also against the bite of rabid dogs. Such a method as this, how- 

 ever, had several disadvantages, and was not absolutely certain in its 

 action, as only fifteen out of twenty dogs were completely protected. 

 Pasteur, therefore, assisted by Chamberland and Roux, devised a more 

 trustworthy and accurate method, in which he utilized the fact that the 

 cord of a rabic animal when kept under certain conditions loses its viru- 

 lence in fourteen days. A series of cords cut into short segments, which 

 were held in series by the dura mater, were suspended in sterile glass 



