566 SMALLPOX AND VACCINATION 



an affected animal were insusceptible to subsequent infection 

 from smallpox. In the horse there occurs a disease known as 

 horsepox, especially tending .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 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 communi- 

 cated 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 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 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 



