(From derivo, to draw from; and from de, and rivus, a river). Derivation. In medicine, when a humour cannot conveniently be evacuated at the part affected, and is attracted from thence, to be discharged at another place, it is called derivation: thus a blister is applied to the neck to draw away the humour from the head.

The doctrine of derivation and revulsion, as understood and explained by the ancients, is, in their sense of these terms, wholly exploded. By revulsion they meant the driving back of the fluids from one part to determine it to another. The only rational meaning that the word revulsion, as here applied, can have, is the preventing too great an afflux of humours to any part, either by contracting the area of the vessels, or diminishing the quantity which flows from them; the first of these intentions is answered by the application of repellents to the part; the last by bleeding and other evacuations. The great object of the older authors was, however, to derive from a part, by establishing a drain in a very distant one. Thus they applied sinapisms to the feet to relieve the head. The fallacy of this reasoning we have noticed under the article of Circulation. Revulsion was a reciprocal term to derivation: for revulsion was, in their sense, made by deriving to a distant part. The language and the ideas remain, though the error has been often demonstrated. It means also the derivation of a word, deducing it from its original source.