The Eucharistic Miracle at Bolsena
The legend at Orvieto concerns a priest in the 14th century in the church of St. Christina in neighboring Bolsena. He had been having doubts about the real presence of Christ in the Eucharist. But one day he was saying Mass and drops of blood fell from the host (the consecrated bread) onto the "corporal," the cloth laid out beneath it. The corporal made its way to Orvieto Cathedral, where a special chapel still houses it today. According to the Catholic Encyclopedia (s.v. "Orvieto"), "a similar legend of the 'blood-stained corporal' is quite frequent in the legendaries of even earlier date than the fourteenth century, and coincides with the great Eucharistic polemics of the ninth to the twelfth centuries."
The fresco shows the red blood both on the corporal and on the right edge of the host. The words along the bottom border are SACERDOS AD ECCLESIA S. CHRISTINAE IN CORPORALI VIDET MIRACULUM, "The priest in the church of St. Christina sees the miracle." The priest's broad tonsure suggests that he belongs to a monastic order.
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Also see the similar story of the Mass of St. Gregory.
Photographed at the cathedral by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.