VACCINES AND SERA 269 



rabid dog taken at random inoculated under the dura mater 

 of the brain of a rabbit after trephining the skull produces 

 rabies in the rabbit after a variable period of incubation ; but 

 after about twenty passages from rabbit to rabbit the incuba- 

 tion period falls to six to seven days ; the virus has then 

 become the virus fixe, and the passages from rabbit to rabbit 

 are really cultures in the living body. The culture exposed to 

 dry air, but sheltered from light, gradually loses some of its 

 virulence. The dried cords, inoculated in the form of emulsions 

 under the skin of animals, produce in them a certain and 

 stable immunity against the strongest virus inoculated under 

 the dura mater. The cultivation in vivo of the %>irus fixe was 

 for Pasteur the key to vaccination against rabies after the bite 

 of the rabid animal. 



There is some doubt whether the virus of the dried cords 

 really undergoes attenuation. Even in 1885 Pasteur said that it 

 was rather a rarefaction which occurred. " The delays observed 

 in the duration of incubation of the rabies communicated day 

 by day to rabbits, to estimate the virulence of our cords dried 

 in contact with air, result from a diminution in quantity of the 

 virus contained, and not from a diminution in virulence." A 

 variation of Pasteur's treatment, the treatment of Hogyes, 

 consists in replacing the graduated desiccation by a graduated 

 dilution.' 



^ The nature of this virus fixe cultivated in the rabbit is rather a 

 question. Nitsch and Marx, who inoculated themselves with fresh virus 

 fixe, consider that after several hundred passages in rabbits the virus 

 becomes harmless to man ; they consider it as a true Jennerian vaccine for 

 man, resulting by adaptation to a different species. But it is not right to 

 maintain the absolute innocuousness of virus fixe. It may well be harm- 

 less when injected into the subcutaneous tissue, but would it continue to be 

 so if injected in a region rich in nerve filaments such as the face or the end 

 of finger ? 



It is probable that the virus of dried cords is destroyed by the body, that 

 the microbe is absorbed, and that protective substances are produced in 

 consequence and are found in the blood of treated individuals. But these 

 substances play only a secondary part, for there is no constant relation 

 between the properties of the serum of an animal and its resistance to 

 rallies. There are certain animals refractory to rabies, such as a species of 

 tortoise whose blood nevertheless possesses no antirabic power. It is 

 impossible to avoid thinking of the phj^ocytes in such a condition. 



