A woman whose husband died without children must marry her late husband’s brother (yibum) or perform the chalitzah ceremony (a sort of divorce from the brother-in-law). It is written, “If brothers dwell together, and one of them dies and have no child, the wife of the dead man shall not marry a stranger: her husband’s brother shall marry her and take her as a wife, and perform the duty of a husband’s brother unto her…Then shall his brother’s wife come unto him in the presence of the elders, and loose his sandal from off his foot and spit in his face” (Deuteronomy 25:5-9). The chalitzah/I> ceremony is accompanied by removing the brother-in-law’s sandal and spitting in his face.
The scholars asked: what is the rule for a widow who has no arms? Must the sandal removal be with her hands, or may she use her mouth and teeth to untie the laces and remove the footwear? They further asked: what is the rule if she spit blood in place of saliva? They went to the study hall and were answered: The Scriptures mentioned the removal, but did not specify how this was to be done, and the Scriptures mentioned the spit, but did not specify what should be spit. As they put it: “Where does it say the removal uses hands? And where does it say that she spit saliva?”
(Babylonian Talmud, Tractate Yevamot 105a)