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Rh In applying to cow-pox the term “variolae vaccinae," Jenner gave expression to his belief that this disease was in reality nothing more nor less than small-pox of the cow. But soon it was discovered that if there were such a malady as “small-pox of the cow,” there was also, as Dr Loy first satisfactorily demonstrated,

a small-pox of the horse, which, under the name of “grease,” was resorted to from time to time as a source of vaccine lymph. Jenner had, indeed, put forward the suggestion that “grease” was a necessary antecedent to cowpox; but even taking this term to have been used by him in the sense of horse-pox, he was, in all probability, mistaken in his assumption. At the same time, however, there can be little doubt that these two diseases are very closely allied, if indeed they be not identical. As evidence of a definite relationship between human small-pox and cow-pox, it may be mentioned that whereas, prior to the introduction of vaccination, epidemics of these disorders frequently arose concurrently, the so-called “natural” cow-pox has now in great measure disappeared. There is, moreover, no appreciable difference in the minute anatomical appearances characteristic of the eruption following on inoculation of one or other of these two affections in the human subject. But of far greater importance in this connexion are the results obtained by numerous observers who, in various parts of the world, and almost from the time of lenner onwards, have set themselves the task of attempting, by experimental methods, to solve the problem of the true relationship of variola to vaccinia. As the outcome of this work it may now be definitely stated that small-pox lymph, more especially, as the present writer has shown, if obtained from the primary vesicle of a case of the inoculated form of the disease, by passage through the system of the calf can be so altered in character as to become deprived of its power of causing a generalized eruption, while inducing at the site of inoculation a vesicle indistinguishable from a typical vaccine vesicle; and, more important still, that when transferred again to man, it has by such treatment completely lost its former infectious character. Such being the case, it may fairly be asserted that cow-pox, or rather that artificially inoculated form of the disease which we term vaccinia, is nothing more nor less than variola modified by transmission through the bovine animal. An outbreak of small-pox, indeed, may be turned to account for raising, by appropriate experimental methods, a fresh stock of vaccine lymph.

There is much evidence to prove that the results following on vaccination are due to a specific contagium, and, moreover, that the particular micro-organism concerned is capable of existing, during one period of its life-cycle, in a resting or spore form, in which condition it is more resistant to the germicidal effects of glycerin than is the case

with non-sporing microbes. Advantage is taken of this fact, in the method devised by the present writer, and now -employed officially in England, as also on the Continent and in America, for ensuring the bacteriological purity of vaccine lymph. Up to the present, unfortunately, no satisfactory method has been discovered by which the micro-organism of vaccinia can be unfailingly cultivated on artificial media while still retaining its specific properties.

The clinical activity and bacteriological purity of the lymph employed for vaccination; the skilful performance of the operation itself; the making an adequate number of insertions of lymph over a sufficient area; the observance of precautions needful for ensuring strict asepsis, both at the time of vaccination and subsequently

until the vaccination wounds are soundly healed—all these are matters to be regarded as essential to “efficient vaccination.” Certain principles in respect of them are generally recognized, and in the case of public vaccinators, whose work comes under government inspection, a series of instructions on these several points are prescribed by the Local Government Board. First in regard to lymph. That which is now almost universally employed in Great Britain is glycerinated cali lymph, the use of which has entirely superseded, in public vaccinations, the arm-to-arm method which for many years previously had been employed as the best means then attainable