326 BACTERtA. 



dura mater (the outer skin or covering of the brain) produce 

 similar effects. Nerves were also found to contain the viru 

 but the saliva, as we have seen, and the salivary glam 

 introduced in the above manner produced a much moi 

 virulent and rapid form of the disease than the other tissue 

 and fluids mentioned : quite as virulent a form, in fact, i 

 when the animal was actually bitten by a rabid animal. 



Finding that these tissues and fluids taken from a rabi 

 animal varied in their virulence, and knowing that in th 

 case of anthrax virus the virulence may be diminished c 

 increased by inoculating into an animal of another specie: 

 he made another series of experiments, as a result of whic 

 he found, that although virus taken from similar position 

 say, the cerebro-spinal fluid, had always the same action i 

 the same species, when the fluid was taken from an anim; 

 of a different species it was weaker or stronger as the ca: 

 might be. Thus in a dog the virus is of constant strengt! 

 and inoculations made from dog to dog kill the animal wit 

 the same incubation period, the same symptoms, and pract 

 cally in the same time. When inoculated from the dog i 

 the monkey, however, the virus becomes less virulent ; 

 is said to be attenuated or weakened, the attenuation b 

 coming more and more marked in successive inoculatioi 

 from monkey to monkey ; the course of the dise^ becom 

 longer and longer, until eventually there m^come a tin 

 at which the virus, when introduced under the skin or inl 

 the cranial cavity, is not sufficiently active to cause the deal 

 of this species. If this attenuated fluid be now inoculate 

 into a rabbit, a dog, or a guinea-pig, it still remains con 

 paratively weak for a time, through successive inoculatioi 

 on these animals — i.e., at first it does not kill, then it kill 

 "but only after a considerable time ; but gradually tl 

 virulence returns, until at length it reaches its original lev 

 of malignancy ; whilst, if the successive inoculations ai 

 made in rabbits with primary fluid from either the dog ( 

 the monkey, the virulence may hecome so exalted that it 

 considerably greater even than that of the virus taken froi 

 the street dog, which at one time was supposed to be tl 

 most virulent form except that of hydrophobic wolves, whic 

 has always been known to be specially fatal ; the virulen( 

 is doubled as the inoculation period is reduced to about or 

 half. 



