504 SMALLPOX AND VACCINATION 



were insusceptible to subsequent infection from smallpox. In the 

 horse there occurs a disease known as horsepox, especially tend- 

 ing to arise in wet cold springs, which consists in an inflammatory 

 condition about the hocks, giving 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 identical with horsepox 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 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. 

 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 Vaccince. Though from the first Jennerian 

 vaccination had many opponents, it gradually gained the con- 

 fidence of the unprejudiced, 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 cowpox, 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 

 inoculated, 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 most of the strains at present in use have probably 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. 



