Carmina Gadelica, Volume 2, by Alexander Carmicheal, [1900], at sacred-texts.com

FLAT moorland is generally intersected with innumerable veins, channels, and ditches. Sometimes these are serious obstacles to cattle, more especially to cows, which are accurate judges. When a cow hesitates to cross, the person driving her throws a stalk or a twig into the ditch before the unwilling animal and sings the 'Feith Mhoire,' Vein of Mary, to encourage her to cross, and to assure her that a safe bridge is before her. The stalk may be of any corn or grass except the reed, and the twig of any wood except the wild fig, the aspen, and the thorn. All these are forbidden, or 'crossed' as the people say, because of their ungracious conduct to the Gracious One. The reed is 'crossed' because it carried the sponge dipped in vinegar; the fig-tree because of its inhospitality; the aspen because it held up its head haughtily, proud that the cross was made p. 133 of its wood, when all the trees of the forest--all save the aspen alone--bowed their heads in reverence to the King of glory passing by on the way to Calvary; and the thorn-tree because of its prickly pride in having been made into a crown for the King of kings. Notwithstanding, however, the wand of safety and the hymn of the herdsman, a cow driven against her will sometimes sinks into the ditch while crossing. This may necessitate the assistance of neighbours to extricate her from her helpless position. Hence the proverb:--'Is e fear na bo fein theid ’s an fheith an tos'--It is the man of the cow himself who shall go into the ditch first. The practice of throwing down the wand and repeating the hymn gave rise to a proverb among the more sceptical of the people:--'Cha dean thu feith Mhoire orm-s’ idir a mhicean'--Thou wilt not make a 'vein of Mary' upon me at all, sonnie.