HYDROPHOBIA. 
149 
the disease from animal to animal by inoculating into 
the nervous system of the well animal a tiny portion 
of nerve tissue from one which had succumbed. The 
inoculated animals invariably died at a fixed period. 
After a long series of studies which we cannot here 
review, he discovered that if the spinal cord of one 
of the inoculated animals (rabbits) which had died 
be dried in a clean place, it gradually lost its virulence, 
so that whereas at first it invariably killed in seven 
days, day by day it lost its power, so that after drying 
for fourteen days it was quite inert. Given thus a 
virus ranging gradually from the very feeble up to 
the strongest, he saw the possibility of gradually ac¬ 
customing the body to the stuff, so that at last it would 
resist the very strongest. 
This was tried on dogs, and it was found that after 
this gradual adaptation to the virus they became at 
last wholly indifferent to the bites of mad dogs or 
the artificial inoculation of the strongest virus. The 
principle was finally applied to man, with the most 
remarkable and satisfactory results. 
Rabies is peculiar in that a long period usually 
elapses between the bite of a rabic animal and the 
development of symptoms. This period, called the in¬ 
cubation period, is in man on the average from thirty 
to forty days; so that if the preventive treatment be 
instituted without undue delay, there is usually time 
for the adaptation of the subject to the artificial virus. 
This accomplished, the disease does not occur. 
