DISEASES OF THE OX AND SHEEP. 295 



animals which have died of hydrophobia, is always virulent. 

 Rabies communicated by intravenous injection of the virus very 

 frequently exhibits characters which differ considerably from 

 those of furious rabies supervening upon a bite, or after trephin- 

 ing, and it is very likely that many cases of silent madness 

 have escaped observation. In such cases of rabies, which 

 might be termed spinal, early paralysis is a common symptom, 

 whilst the habitual fury and rabid barks are absent or but 

 rarely met with ; but, on the other hand, frightful itching of the 

 skin is at times a marked feature. 



Experiments show that after inoculation of the poison into the 

 blood-system, the spinal marrow is the part first attacked, the 

 virus locating itself and multiplying there before spreading to 

 other parts. On the 19th May, 1884, about the fourth year of 

 Pasteur's research, he communicated the following observations 

 to the Academy of Sciences. 



"The virus of rabies carried from the dog to the monkey, and 

 subsequently from monkey to monkey, grows weaker at each 

 passage. After the virulence has thus diminished by several 

 passages through monkeys, if the virus be carried back to the 

 dog, the rabbit, or the guinea-pig, it still remains attenuated. 

 In other words, the virulence does not go back at one bound to 

 the degree it had in the dog affected with ordinary or street 

 madness. On the other hand, successive passages from rabbit 

 to rabbit, and from guinea-pig to guinea-pig, increase the viru- 

 lence of rabies' virus. This exalted virulence comes to a fixed 

 maximum in the rabbit. If now transferred to the dog, it 

 remains exalted, and shows itself to be much more intensely 

 virulent than the virus of ordinary street rabies. So great is 

 this acquired virulence, that the new virus injected into the 

 blood-system of a dog unfailingly gives rise to mortal 

 madness." 



A logical application of the above results gives us the means 

 of easily rendering dogs refractory to rabies, for we can now 

 prepare and keep at our disposal a series of attenuated viruses 

 of different strengths, some not mortal, preserving the animal 

 economy against the ill-effects of more active ones, and these 

 latter against the effects of mortal ones. 



It was not long before Pasteur found that there was no 

 definite period of incubation, in cases in which the saliva of the 



