From St. Gregory of Nyssa's Catechism:
The question was, how can that one Body of Christ vivify the whole of mankind, all, that is, in whomsoever there is Faith, and yet, though divided amongst all, be itself not diminished? Perhaps, then, we are now not far from the probable explanation. If the subsistence of every body depends on nourishment, and this is eating and drinking, and in the case of our eating there is bread and in the case of our drinking water sweetened with wine, and if, as was explained at the beginning, the Word of God, Who is both God and the Word, coalesced with man’s nature, and when He came in a body such as ours did not innovate on man’s physical constitution so as to make it other than it was, but secured continuance for His own body by the customary and proper means, and controlled its subsistence by meat and drink, the former of which was bread,—just, then, as in the case of ourselves, as has been repeatedly said already, if a person sees bread he also, in a kind of way, looks on a human body, for by the bread being within it the bread becomes it, so also, in that other case, the body into which God entered, by partaking of the nourishment of bread, was, in a certain measure, the same with it; that nourishment, as we have said, changing itself into the nature of the body. For that which is peculiar to all flesh is acknowledged also in the case of that flesh, namely, that that Body too was maintained by bread; which Body also by the indwelling of God the Word was transmuted to the dignity of Godhead. Rightly, then, do we believe that now also the bread which is consecrated by the Word of God is changed into the Body of God the Word. For that Body was once, by implication, bread, but has been consecrated by the inhabitation of the Word that tabernacled in the flesh. Therefore, from the same cause as that by which the bread that was transformed in that Body was changed to a Divine potency, a similar result takes place now. For as in that case, too, the grace of the Word used to make holy the Body, the substance of which came of the bread, and in a manner was itself bread, so also in this case the bread, as says the Apostle 1 Tim. iv. 5., “is sanctified by the Word of God and prayer”; not that it advances by the process of eating. Seeing, too, that all flesh is nourished by what is moist (for without this combination our earthly part would not continue to live), just as we support by food which is firm and solid the solid part of our body, in like manner we supplement the moist part from the kindred element; and this, when within us, by its faculty of being transmitted, is changed to blood, and especially if through the wine it receives the faculty of being transmuted into heat. Since, then, that God-containing flesh partook for its substance and support of this particular nourishment also, and since the God who was manifested infused Himself into perishable humanity for this purpose, viz. that by this communion with Deity mankind might at the same time be deified, for this end it is that, by dispensation of His grace, He disseminates Himself in every believer through that flesh, whose substance comes from bread and wine, blending Himself with the bodies of believers, to secure that, by this union with the immortal, man, too, may be a sharer in incorruption. He gives these gifts by virtue of the benediction through which He transelements.
I am not usually a fan of Thomistic theology, but I do love the Saint for his devotion and understanding of the Eucharist. Indeed, in my opinion his greatest contribution to the Church is not the Summa but the Corpus Christi Office, Mass, hymns, and benediction. Very similar language and description of St. Gregory's understanding of the Eucharist can be found in St. Thomas's Pange Lingua: "Verbum caro, carnem verum/ Verbo carnem effect."
This feast is an opportune moment to remember that the Church is the Body of Christ in a very literal way, and that the Eucharist, along with Baptism, is the means by which Christ makes that happen. The Church is God sharing Himself with mankind, a loving condescension that should make us blush, but not be scandalized.
I will save you from my poor observations, but I have put the antiphons for first Vespers of Corpus Christi below in English. They are worth some meditation (hint: pray Vespers tonight here).
Psalm 109: Christ the Lord, being made an High Priest for ever after the order of Melchisedech, hath offered bread and wine
Psalm 110: He hath made His wonderful works to be remembered; the Lord is gracious and full of compassion. He hath given meat unto them that fear Him
Psalm 115: I will take the cup of salvation, and offer the sacrifice of thanksgiving
Psalm 127: Let the children of the Church be like olive-plants round about the table of the Lord
Psalm 147: The Lord, That maketh peace in the borders of the Church, filleth her with the finest of the wheat