568 SMALLPOX AND VACCINATION 



after passage through a series of monkeys, a virus of attenuated 

 but constant virulence can be obtained. We have seen there 

 are good grounds for believing that the virus of calf lymph 

 confers immunity against human smallpox. In considering the 

 relationships of cowpox and smallpox, this is an important 

 though subsidiary point ; for at present it is questionable 

 whether there are any well-authenticated instances of one 

 disease having the capacity of conferring immunity against 

 another. The most difficult question in this connection is what 

 happens when inoculations of smallpox matter are made on 

 cattle. Chauveau denies that in such circumstances cowpox is 

 obtained. He, however, only experimented on adult cows. The 

 transformation has been accomplished by many observers, 

 including, in this country, Simpson, Klein, Hime, and Copeman. 

 The general result of these experiments has been that if a series 

 of calves is inoculated with variolous matter, in the first there 

 may not be much local reaction, though redness and swelling 

 appear at the point of inoculation, and some general symptoms 

 manifest 'themselves. On squeezing some of the lymph from 

 such lesion as occurs, and using it to continue the passages 

 through other calves, after a very few transfers a local reaction 

 indistinguishable from that caused by cowpox lymph generally 

 takes place, and the animals are now found to be immune 

 against the latter. Not only so, but on using for human 

 vaccination the lymph from such variolated calves, results 

 indistinguishable from those produced by vaccine lymph are 

 obtained, and the transitory illness which follows, unlike that 

 produced in man by inoculation with smallpox lymph, is no 

 longer infectious. In fact, many of the strains of lymph in use 

 in Germany at present have been derived thus from the variola- 

 tion of calves. The criticism of these experiments which has 

 been offered, namely, that since many of them were performed 

 in vaccine establishments, the calves were probably at the same 

 time infected with vaccinia, is not of great weight, as in all the 

 recent cases at least, very elaborate precautions have been 

 adopted against such a contingency. And at any rate it 

 would be rather extraordinary that this accident should happen 

 in every case. We can, therefore, say that at present there 

 is the very strongest ground for holding not only that vaccinia 

 confers immunity against variola, but that variola confers 

 immunity against vaccinia. The experimentum crucis- for 

 establishing the identity of the two diseases would of course 

 be the isolation of the same micro-organism from both, and the 

 obtaining of all the results just detailed by means of pure 



