490 Eabies. 



With the increase in virulence for rabbits, a similar increase does 

 not seem to follow for other animals and for human beings. Thus 

 Helman injected 1-10 g. of fixed virus into 34 dogs and 8 monkeys 4 

 days in succession without rabies appearing in a single animal ; further, 

 Nitzsch inocuUted himself under the skin with an emulsion from a 

 4-5 mm. piece of rabbit cord without experiencing any bad results what- 

 ever, and also found the fixed virus without effect on dogs and 

 monkeys. 



After securing a rabies virus of known and constant viru- 

 lence, Pasteur set to work to obtain its attenuation. Having bad 

 to discontinue the passage through monkeys, owing to its high 

 cost, he chose the following procedure : 



The medulla and spinal cord of a rabbit, which has contracted 

 rabies after a subdural injection with fixed virus, are suspended free 

 by means of threads in a bottle closed by a plug of cotton and the bottom 

 of which is covered with pieces of caustic potash. At a constant tem- 

 perature of 23° C, the spinal cord will dry, and also gradually lose 

 its virulent properties, so that, for instance, spinal cord dried for 5 

 days will produce the disease on the 8th day, cord dried for 9 days 

 on the 15th day, and the same dried for 14 days will be absolutely without 

 effect. If spinal cord from a rabbit dead of rabies is treated in this 

 manner daily for 14 days, there will be, after 2 weeks, material on hand 

 of 14 different degrees of virulence. 



Pasteur then immunized dogs by injecting at first the most 

 attenuated virus, and then every second hour, a virus grad- 

 ually less attenuated until he reached spinal cord which had 

 been dried only one day. Dogs which had been subjected to 

 2-3 series of such inoculations were later bitten by a rabid dog, 

 and were also injected subdurally with fixed virus or street 

 virus, without suffering harm. 



Through further experiments, Pasteur proved that this 

 method of immunizing likewise prevented the appearance of the 

 disease in dogs previously bitten by rabid animals. The im- 

 munity obtained by protective vaccination lasts about one year, 

 but is still effective after two years insofar as rabies develops 

 after a considerably longer period following inoculation (Pas- 

 teur). In exceptional cases the immunity may be effective up 

 to 5 years (Hogyes) ; it may be further prolonged by vaccina- 

 tions repeated at intervals. 



Finally, in July, 1885, Pasteur successfully applied the 

 method, which he had proved to be effective in dogs, to a boy 

 who had been bitten. Since then, the anti-rabies protective 

 vaccination has always been practiced on human beings in Pas- 

 teur Institutes established for this purpose, either according to 

 Pasteur's or to a somewhat modified method. 



I. Protective Vaccination with Dried Substance of Spinal Cord. 



(Pasteur's method.) The method which is nearly always followed in 

 human beings consists in taking 2 mg. of the dried substance of the 

 spinal cord of rabbits dead of rabies after inoculation with fixed virus, 



