2 S 4 THE POPULAR SCIENCE MONTHLY. 



the occurrence of small-pox is about the same as that of a first attack 

 of small-pox against its recurrence, suspected that cow-pox might really 

 be small-pox modified by passing through the living body of the cow ; 

 and attempts have been made, at different times and in various places, 

 to test the truth of this hypothesis. Before proceeding, however, to 

 discuss that question, it will be advantageous to consider what new 

 lio-ht is cast, by recent scientific discovery, on the nature of the protec- 

 tion afforded by successful vaccination. 



Notwithstanding the " strong assurance of faith," on the part of 

 Jenner and his immediate disciples, in regard to the permanent efficacy 

 of vaccination, it is certain that, as time went on, a suspicion grew up 

 among vaccinators of long experience, that vaccinia has a tendency to 

 degenerate i. e., to lose its protective power in proportion to the 

 remoteness of its derivation from the original (cow) stock. During 

 my own early professional life (1830-40) in Bristol, this conviction 

 was prevalent among the older practitioners, who recollected the early 

 Jennerian cow-pock. The vesicle (they said) was smaller than the 

 original, and ran its course more quickly ; and the want of the slight 

 constitutional disturbance formerly observable at its maturity showed 

 that the body of the subject was not thoroughly affected by the 

 disorder. Hearing in 1838 of a renewed outbreak of cow-pox among 

 cows at Berkeley, Mr. J. B. Estlin (whose pupil I had been) went down 

 thither, and brought back a supply of original vaccine lymph, which 

 (with the assistance of his brother practitioners) was soon diffused 

 through Bristol and its neighborhood, and proved to reproduce the 

 characteristic Jennerian vesicle. The circumstances attending this rein- 

 troduction of an original vaccinia, which I have recently detailed else- 

 where,* strongly impressed me with the idea that the vaccine virus 

 became " tempered " (so to speak) by passing through the human body, 

 its original potency suffering diminution with the increase in the num- 

 ber of subjects through which it had been transmitted ; while, at the 

 same time, the proportion of subjects in whom the vaccination " took," 

 which had been small with the original " vaccine," increased when it 

 had (so to speak) become " humanized." This gradual modification 

 we now understand to be the natural result of the continued " cultiva- 

 tion " of vaccinia in the human body ; so that the diminution of the 

 protective power of vaccination by such " cultivation " through a long 

 succession of generations is just what might be scientifically expected. 

 A most curious proof of the modification which vaccinia, thus human- 

 ized, has undergone, is afforded by the experiments of Dr. Martin (of 

 Foxborough, Massachusetts), who states that, while there is no diffi- 

 culty in keeping up an original vaccinia for any length of time by 

 continuous transmission through heifers, the humanized vaccinia, if 

 recommuuicated to heifers, soon dies out, this retro-vaccination (as 



* Sec the "Lancet," May 10th. 



