502 APPENDIX A. 



dressed the sores to the teats of cows-subsequently milked by them, gave 

 :rise to cowpox in the latter. This disease was thus identical with horse- 

 pox, in epidemics of which it had its origin. Jenner was, however, 

 probably in error in confounding horsepox with another disease of 

 horses, namely, " grease." Cowpox manifests itself as a papular eruption 

 on the teats ; the papules become pustules ; their contents dry up to 

 form scabs, or more or less deep ulcers are formed at their sites. From 

 such a lesion the hands of the milkers may become infected through 

 abrasions, and a similar local eruption occurs, with general symptoms in 

 the form of slight fever, malaise, and loss of appetite. It is this illness, 

 which, according to Jenner, gives rise to immunity from smallpox infec- 

 tion. He showed experimentally that persons who had suffered from 

 such attacks did not react to inoculation with smallpox, and further, that 

 persons to whom he communicated cowpox artificially, were similarly 

 immune. The results of Jenner's observations and experiments were 

 published in 1 798 under the title An Inquiry info the Causes and Effects 

 of the Variola Vaccines. Though from the first Jennerian vaccination 

 had many opponents, it gradually gained the confidence of the unpreju- 

 diced, and became extensively practised all over the world, as it is at 

 the present day. 



The evidence in favour of vaccination is very strong. There is no 

 doubt that inoculation with lymph properly taken from a case of cow- 

 pox can be maintained with very little variation in strength for a long 

 time by passage from calf to calf, and such calves are now the usual 

 source of the lymph used for human vaccination. When lymph derived 

 from them is used for the latter purpose, immunity" against smallpox is 

 conferred on the vaccinated individual. It has been objected that some 

 of the lymph which has been used has been derived from calves inocu- 

 lated, not with cowpox, but with human smallpox. It is possible that 

 this may have occurred in some of the strains of lymph in use shortly 

 after the publication of Jenner's discovery, but there is no doubt that 

 most of the strains at present in use have been derived originally from 

 cowpox. The most striking evidence in favour of vaccination is derived 

 from its effects among the staffs of smallpox hospitals, for here, in 

 numerous instances, it is only the unvaccinated individuals who have 

 contracted the disease. While vaccination is undoubtedly efficacious in 

 protecting against smallpox, Jenner was wrong in supposing that a vac- 

 cination in infancy afforded protection for more than a certain number 

 of years thereafter. It has been noted in smallpox epidemics which 

 have occurred since the introduction of vaccination, that whereas young 

 unprotected subjects readily contract the disease, those vaccinated as 

 Infants escape more or less till after the thirteenth to the fifteenth years. 

 It has become, therefore, more and more evident that revaccination is 



