No 6 DEPARTiMENT OK AGRICULTURE. 527 



beginning with the mildest virus and going gradually up to one 

 which possesses full virulence. Tfiis attenuation of the virus is 

 brought about by drying at a fixed temperature, and the action of the 

 atmosphere. Depending upon the length of time the virus is ex- 

 posed to these influences, we can obtain any degree of virulence de- 

 sired, the loss of virulence under fixed conditions being quite uni- 

 form. 



The disease as seen in dogs infected naturally was called by Pas 

 teur "street rabies," and the virus from such animals is known 

 as the ''virus of street rabies.'' Such virus will produce the dis- 

 ease in rabbits by intracranial inoculation in from three to four 

 weeks as a rule. By inoculating rabbits in series, one from the 

 other, we obtain a reduction in the period of incubation so that 

 after about 100 passages, rabbits will die on the sixth or seventh day 

 after inoculation with great certainty. Beyond this point no in- 

 crease of virulence can be obtained, therefore, to this virus the 

 nam(^ of "fixed" was given. It is with this "fixed" virus that all 

 of our methods of vaccination are carried out. 



Preparation of Vaccine. The spinal cord of a rabbit which has 

 died after inoculation with this "fixed" virus is I'emoved with the 

 greatest precaution to prevent contamination. It is then cut into 

 three portions, of equal length, and suspended by a silk thread in 

 a large bottle containing a layer of caustic potash at the bottom. 

 The bottle has an opening near the bottom as well as at the top, both 

 of which are plugged with cotton, to allow a free passage of atmos- 

 pheric air. Tliese are kept in a dark room at a carefully main- 

 tained temperature of 23 degrees C. Under these conditions the 

 cords lose their virulence entirely in about fifteen days. Those kept 

 for fourteen days have a very slight degree of virulence, and fur- 

 nish our weakest virus with which the vaccination is begun. On 

 succeeding days cords which have been dried for thirteen, twelve, 

 eleven, etc., days, are used, until one which has unimpaired viru- 

 lence may be injected with perfect safety. The method was tried 

 at first on animals with perfect success, and since 1885 has been 

 applied to persons bitten by rabid animals. The treatment is varied 

 slightly according to the extent of the bites and their location. The 

 simplest form requires fifteen days, the next eighteen days, while 

 what is called the "intensive" treatment, employed where the bites 

 are about the- face, and in portions of the body rich in nerve supply, 

 requires twenty-one days. The one most commonly used is given 

 below: 



