296 THE DISEASES AND DISORDERS OP THE OX. 



mad dog entered the venous system in the ordinary way. If a 

 large dose of the poison entered, the period of incubation would 

 be short. If only a small quantity entered, it might become 

 localised at the seat of inoculation and wither away, or on a 

 more favourable soil it might slowly and surely work its way 

 and develop into madness months later. The difficulty was to 

 find artificial certainty. If the brains of rabbits were inoculated 

 with the spinal marrow of an ordinary mad dog, Pasteur found 

 that the disease was usually fatal on the fifteenth day. But if 

 another rabbit were inoculated from the first, a third from the 

 second, and so on, the period of incubation would gradually 

 diminish with the increasing virulence of the poison. When 

 the number of passages from rabbit to rabbit extended to the 

 twenty-fifih rabbit, the period of incubation was shortened down 

 to eight days, and after the next twenty-five passages from 

 rabbit to rabbit it became shortened to seven days. At the 

 ninetieth passage the maximum of virulence was attained with 

 the corresponding incubation of seven days, and with a cer- 

 tainty which would be rendered absolute by preserving the 

 rabbits in perfect health and by taking them at a uniform age 

 of six months. 



It was found that not one out of twenty-three vaccinated dogs 

 which had been bitten by ordinary mad dogs had taken rabies. 

 On the other hand, within two months after the bites, 66 per 

 cent, of the control dogs which had been similarly bitten had 

 become afi'ected with rabies. 



According to M. Pasteur, rabies is mainly and essentially a 

 disease of nerve centres. The virus or infective material exists 

 more especially in the medulla oblongata, in certain parts of the 

 brain, and in the spinal cord. The same observer has shown 

 that the injection of a few drops of sterilised fluid containing a 

 small portion of the medulla oblongata of a dog which has died of 

 rabies is sure to cause the appearance of the disease after a period 

 of incubation of about fourteen days. Similarly, the inoculation 

 of a rabbit by trepanning under the dura mater with the rabical 

 marrow of a mad dog always causes the production of rabies in 

 the rabbit after about a fortnight has elapsed. Now, if the virus 

 of this rabid rabbit is passed on to a second, and then if that of 

 this second is passed on to a third, and so on by the same mode 

 of inoculation as above-mentioned, the period of incubation 



