RABIES 305 



of the world. The method is based upon the principle of stimulat- 

 ing the production of rabic antibodies by the injection of weakened 

 virus during the period of incubation, so that the virus introduced 

 into the wound may be destroyed. Starting with the idea that 

 the virus as it occurs in rabid dogs under natural conditions (street 

 virus) is of a more or less constant degree of virulence, he found 

 that the potency of such a virus could be diminished within a cer- 

 tain limit by passage through monkeys and similarly increased 

 by passage through rabbits. In the latter case the virulence 

 could be so exalted for rabbits that the incubation period was 

 lessened from twelve or fourteen days to six or seven days, but 

 beyond that point it was impossible to go. A virus of such strength 

 he termed " fixed." By drying fixed virus over caustic potash 

 he was able to obtain different degrees of attenuation according 

 to the length of time of exposure, the weakest virus being so modi- 

 fied that it could not produce the disease in man but yet was able 

 to produce the specific antibodies. 



" Fixed " virus is prepared by the subdural inoculation of street 

 virus through a series of young rabbits. During from thirty to 

 fifty passages the incubation period is reduced to six or seven 

 days. Immediately on the death of the animal a piece of the 

 floor of the fourth ventricle is emulsified in sterile broth and 

 three or four drops of the emulsion are injected beneath the dura 

 of a normal rabbit. In from six to seven days paralytic symptoms 

 appear followed in from three to four days by death. The cord 

 and brain are then removed under aseptic precautions and the 

 brain is set aside for further animal inoculations. From the cord, 

 which is severed just below the medulla, a fragment is cut off and 

 dropped into sterile broth to test for purity and the remainder 

 is then divided into two equal portions which are suspended by 

 sterilized silk threads in a glass jar containing sticks of caustic 

 potash. The jars with the suspended cords are kept in a dark 

 room at a temperature of about 22 C. After a suitable period of 

 drying small pieces of cord are emulsified and injected subcutane- 

 ously into patients under treatment. In the New York Board 

 of Health Laboratories c.c. of the indicated cord is emulsified 



