Hippolytus, Refutation of All Heresies IV.4.3:


This is the way they make sheep cut off their own heads. Secretly anointing the sheep’s throat with a caustic drug, he fixes near the beast a sword and leaves it there. But the sheep, being anxious to scratch himself, leans (heavily) on the knife, rubs himself along it, kills himself and must needs almost cut off his head. And the drug is bryony and marsh salt and squills in equal parts mixed together. So that he may not be seen to have the drug with him, he carries a horn box made double, the visible part of which holds frankincense and the invisible the drug. And he also puts quicksilver into the ears of the animal that is to die. But this is a death-dealing drug. (trans. F. Legge)