SUPPLEMENTARY APPENDIX. 



Experience, and experience alone, can answer that question. A priori we 

 -do not see that there is any better reason for supposing that it would last 

 for two or three years than that its duration would extend to ten years. 



Cow-pox and Small-pox not Convertible. 



Jenner himself, in his first paper, advanced the view that the cow-pox 

 and small-pox were identical with each other ; and since his time 

 numerous observers have attempted to prove the identity of the two 

 diseases experimentally namely, by giving rise to cow-pox in the cow 

 through the inoculation of small-pox matter, or by the introduction of 

 contagion in other ways. It may at once be stated that while cow-pox 

 is readily transferred from the cow to man and back again from man to 

 the cow, the disease in man being identical with that in the cow, small- 

 pox cannot be transferred from man to the cow so as to give rise to a 

 disease in the latter identical in its features with the small-pox of man. 

 ^Nor can cow-pox be so transferred to man as to give rise in him to 

 small-pox. The two diseases are not in this sense convertible. 



Small-pox Vaccine. 



In most cases the attempt to transfer small-pox from man to the 

 cow has had simply a negative result ; no obvious effect of any kind has 

 been observed. This has been the case in the attempts to introduce the 

 contagion through absorption by the respiratory or digestive organs, and 

 in most of the attempts to introduce the contagion by inoculation. In 

 certain instances these latter attempts have produced results which may 

 be briefly described in three categories. (We may pass over the isolated 

 experience of Thiele, who in 1838 asserted that by keeping small-pox 

 matter sealed between glass plates for ten days before using it, and 

 by diluting it with milk when using it for inoculation, the matter thus 

 treated through ten removes through the human body the cow not 

 intervening at all was converted into something which gave results 

 identical with those of ordinary vaccine matter. We are not aware of 

 any attempt to corroborate this experiment.) 



The first category includes the experiments in which the inoculation 

 of small-pox matter into the udder, or adjoining parts, of the cow gave 

 rise at or near the seat of inoculation to a vesicle, either identical in 

 visible characters with the ordinary vaccine vesicle produced by inocula- 

 tion with the matter of cow-pox, or to a vesicle the features of which, 

 while not corresponding wholly with those of a perfect vaccine vesicle, 

 so closely resembled them as to justify the vesicle being called a vaccine 

 vesicle. Further, the matter from a vesicle which at the first inoculation 

 had not the characters of a perfect vaccine vesicle, when carried through 

 a second or third remove in the cow, fully acquired those characters, and 

 when transferred to man gave results indistinguishable from the ordinary 

 vaccine vesicle. Indeed, lymph of such an origin has come into general 

 use for vaccination purposes. Of the experiments, the best known or 

 most quoted are those of Thiele (1838), Ceely (1840), Badcock (between 

 1840 and 1860), Voigt (1881), Haccius and Eternod (1890), King (1891), 



