JENNEKIAN VACCINATION 605 



rise to ulceration. Jenner believed that the matter from these 

 ulcers, when transferred by the hands of men who dressed the 

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

 rise to cowpox in the latter. This disease was thus, in his 

 opinion, identical with horsepox in epidemics of which it had its 

 origin. Cowpox manifests itself as. a papular eruption on the 

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

 form scab's, or more or less deep ulcers occur 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 infection. 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. 1 The results of Jenner's 

 observations and experiments were published in 1798 under 

 the title, An Inquiry into the Causes and Effects of the 

 Variola Vaccinae. Though from the first Jennerian vaccina- 

 tion had many opponents, it gradually gained the confidence of 

 the unprejudiced, and became extensively practised all over the 

 world, as it is at the present day. 



The so-called vaccine lymph which contains the protecting 

 agent is the serous exudate of the cowpox vesicle. When such 

 lymph is used for inoculating calf from calf by passage a con- 

 tinuous supply of a product of very constant potency is obtained ; 

 this is the usual source of the lymph used for human vaccina- 

 tion. By its use 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 

 inoculated, not with cowpox, but with human smallpox. It is 

 possible that this may have occurred in some of the strains in 

 use shortly after the publication of Jenner's discovery, but most 

 of the modern strains have probably been derived originally 

 from cowpox. The most striking evidence in favour of vaccina- 

 tion' 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 vaccination in 

 infancy afforded protection for more than a certain number of 

 years thereafter. It has been noted in smallpox epidemics that 

 whereas young unprotected subjects readily contract the disease, 



