St. Thomas Aquinas on Eucharistic Miracles

It sometimes happens that such apparition comes about not merely by a change wrought in the beholders, but by an appearance which really exists outwardly. And this indeed is seen to happen when it is beheld by everyone under such an appearance, and it remains so not for an hour, but for a considerable time; and, in this case some think that it is the proper species of Christ’s body. Nor does it matter that sometimes Christ’s entire body is not seen there, but part of His flesh, or else that it is not seen in youthful guise, but in the semblance of a child, because it lies within the power of a glorified body for it to be seen by a non-glorified eye either entirely or in part, and under its own semblance or in strange guise...

While the dimensions remain the same as before, there is a miraculous change wrought in the other accidents, such as shape, color, and the rest, so that flesh, or blood, or a child, is seen. And, as was said already, this is not deception, because it is done to represent the truth, namely, to show by this miraculous apparition that Christ’s body and blood are truly in this sacrament.

— St. Thomas Aquinas, Summa Theologica, Part III, Question 76: Of the Way in Which Christ is in This Sacrament, Article 8