EC, in medieval stories you often find characters swearing by bits of their favourite saints- "By St Euphemia's little finger!" etc. Did this in fact happen- was it a medieval way of diluting a curse by not actually using the names of God or Christ?
I don't know that one Annis - though I probably know someone who does and I can ask her next time I speak to her. I'd be interested to know myself! I've not read that many translations of primary sources not written by clerics and men of the church - who are less likely to insert such blasphemies. In the Histoire de Guillaume le Mareschal it's mostly God and Christ who get sworn by. John Marshal tells the Empress Matilda to get her legs astride the horse before they're outrun. 'So help me Christ.' 'Si m'ait Jesu Crist.' Henry II says 'Par les ielz Dieux!' or 'By God's eyes!' Richard the Lionheart says 'Por les gambes Dieu!' or 'By God's legs!'
As far as I recall there is no swearing by saintly parts at all in this particular primary source, it's all Big Deity body parts.