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Reverend Fr. Roberto Coggi, O.P.:
Medieval theologians have carefully examined the matter of Eucharistic Miracles (which were very frequent in their day) and have given various interpretations; but the best founded and most reasonable one seems to be that of St. Thomas Aquinas, the “Eucharistic Doctor” par excellence (Summa Theolgica III, q. 76, a. 8).
He says that the body and blood which appear after a miracle are due to the transformation of the Eucharistic species, namely, of the accidents, and do not touch the true substance of the body and blood of Christ. In other words, the species of bread and wine are miraculously changed into the species of flesh and blood; but the true body and true blood of Jesus are not those which appear but those which, even before the miracle, were hidden beneath the species of flesh and blood.
If, in fact, the flesh and blood which appear were truly the flesh and blood of Christ, we would have to say that the risen Jesus, who reigns impassibly at the right hand of the Father, loses a part of his flesh and blood, something which in no way can be accepted.
We must therefore say that the flesh and blood which appear in the miracles are in the order of species or appearances, neither more nor less than the order of the species of bread and wine.
The Lord performs these miracles to give us a sign, easy and visible to all, that in the Eucharist there is the true body and true blood of the Lord.
But this true body and this true blood are not those that appear, but rather those that are substantially contained under the species or appearances, species and appearances that, before the miracle, were those of bread and wine, and after the miracle are those of flesh and blood.
Under the appearances of flesh and blood Jesus is truly and substantially contained as He was before the miracle. For this reason we can adore Jesus truly present under the species of flesh and blood.