Mitchell brings these examples of Nordic medicine up to raise the point of whether they constitute "magic." The question he does not raise is whether they are Nordic. I have my doubts about whether this is particularly pre-Christian Nordic because it appears elsewhere in a standard version that was known in Scandinavia among the Christians.One recommends that the gall of a black dog, or of a particular kind of fish, be burned in a wooden vessel and used as a remedy against witchcraft and diabolical influences. A Danish formula against wantonness calls for a potion made of the leaves of a plant mixed with liquid, although what makes it stand out is the instruction that the brew should be blended while the Pater noster is being "read." Modern readers naturally assume that recipes of this sort are a form of magic.
But how would medieval users have understood such practices? If a petitioner were to intone phrases invoking a pagan god or the Christian deity while burning the fish gall, for example, would we understand it as a religious practice? Or if the fumes of gall were believed to have a specific chemical or pharmacological purpose, should we then understand this practice as a kind of primitive science? And are the answers we give to these queries merely elusive matters of perspective, or are they in fact hard and fast conclusions, as true in the modern context as they would have been in pre-Christian Scandinavia?
(Stephen A. Mitchell, Witchcraft and Magic in the Nordic Middle Ages [Philadelphia: University of Pennsylvania Press, 2011], 25-26.)
Burning the gall of a particular kind of fish to drive out demons is found in the book of Tobit. The book of Tobit is considered one of the apocryphal books because it is found in the Septuagint but not in the Hebrew Bible. It was, however, considered biblical in the Catholic church in medieval times. The modern Icelandic version of the passage in question (Tobit 6:7-9) is:
Og ungi maðurinn sagði við engilinn: “Bróðir Asaría, til hvers er hjartað, lifrin og gallið úr fiskinum?” Og hann svaraði: “Hjartað og lifrin eru til þess, að ef ári eða illur andi þjáir enhvern, þá á að brenna því sem reykelsi fyrir manninum eða konunni, þá verða þau ekki framar þjáð.”(Those looking for a translation can see here.) Here the angel specifically says that if someone is suffering from a demon or evil spirit that burning the guts of a fish in front of the afflicted man or woman will drive away the demon. (It worked in Tobit's case.) Here we see a Hellenistic Near Eastern folk practice becoming a medieval Scandinavian one.