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 light is cast, by recent scientific discovery, on the nature of the protection 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 reintroduction of an original vaccinia, which I have recently detailed elsewhere, 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 number 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 "cultivation" 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 humanized, has undergone, is afforded by the experiments of Dr. Martin (of Foxborough, Massachusetts), who states that, while there is no difficulty in keeping up an original vaccinia for any length of time by continuous transmission through heifers, the humanized vaccinia, if recommunicated to heifers, soon dies out, this retro-vaccination (as
- See the "Lancet," May 10th.