"Prayer having been offered up, every one saluted the Holy Gospels, the
venerated cross and image of our Lord and Saviour Jesus Christ, and of
our Lady the mother of God, and placed his hands upon them in
confirmation of what he had said."
From these I infer that the custom of swearing on the Gospels received the
sanction of the church at a very early period.
In reply to the question as to other modes of swearing, it may be said
briefly, that men swore by anything to which they attached any importance,
and generally by that to which they attached most importance.
By the laws of the Alamanni, a wife could claim her _Morgen-gabe_ (or the
gift of the morning after the wedding night) by swearing to its amount on
her breast; and by the Droits d'Augsbourg, by swearing to it on her two
breasts and two tresses.
Nothing was more common than for a man to swear by his beard. This custom
is alluded to by one of Shakspeare's fools, who suggests that if a certain
knight swore by his honour, and his mistress by her beard, neither of them
_could_ be forsworn.