504 APPENDIX A. 



merits has been that if a series of calves is inoculated with variolous 

 matter, in the first there may not be much local reaction, though red- 

 ness and swelling appear at the point of inoculation, and some general 

 symptoms manifest themselves. On squeezing some of the lymph from 

 such reaction 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 ani- 

 mals 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 variolation 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 prob- 

 ably at the same time infected with vaccine, 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 to occur 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 experi- 

 mentum 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 cultures or 

 the products of such. In the absence of this evidence we are at 

 present justified in considering that there is strong reason for believing 

 that vaccinia and variola are the same disease, and that the differences 

 between them result from the relative susceptibilities of the two species 

 of animals in which they naturally occur. 



With regard to the relation of cowpox to horsepox, it is extremely 

 probable that they are the same disease. Some epidemics of the former 

 have originated from the horse, but in other cases such a source has not 

 been traced. Cattle-plague, from the clinical standpoint, and also from 

 that of pathological anatomy, resembles very closely human smallpox. 

 Though each of the two diseases is extremely infectious to its appro- 

 priate animal, there is no record of cattle-plague giving rise to smallpox 

 in man or vice versa. When matter from a cattle-plague pustule is 

 inoculated in man, a pustule resembling a vaccine pustule occurs, and 

 further, the individual is asserted to be now immune to vaccination ; 

 but vaccination of cattle with cowpox lymph offers no protection against 

 cattle-plague, though some have looked on the latter as merely a malig- 



