St. Thomas Aquinas on the Eucharist
A Few Passages from the Summa, III
Q. 74, Art. 1. "Bread and wine are the proper matter of this sacrament. And the reasonableness of this is seen, first, in the use of this sacrament, which is eating: for, as water is used in the sacrament of Baptism for the purpose of spiritual cleansing, since bodily cleansing is commonly done with water; so bread and wine, wherewith men are commonly fed, are employed in this sacrament for the use of spiritual eating."
Q. 75, Art. 2, ad 3. "Christ's body is not in this sacrament in the same way as a body is in a place, which by its dimension is commensurate with the place; but in a special manner which is proper to this sacrament. Hence we say that Christ's body is upon many altars, not as in different places, but sacramentally: and thereby we do not understand that Christ is there only as in a sign, although a sacrament is a kind of sign; but that Christ's body is here after a fashion proper to this sacrament."
Q. 75, Art. 4. "Yet this change is not like natural changes, but is entirely supernatural, and effected by God's power alone. . . Hence this is not a formal, but a substantial conversion: nor is it a kind of natural movement: but, with a name of its own, it can be called transubstantiation."
Q. 75, Art. 5. "It is evident to sense that all the accidents of the bread and wine remain after the consecration. And this is reasonably done by Divine providence. First of all, because it is not customary, but horrible, for men to eat human flesh, and to drink blood. And therefore Christ's flesh and blood are set before us to be partaken of under the species of those things which are the more commonly used by men, namely, bread and wine."
Q. 76, Art. 1. (Whether the Whole Christ is Contained under This Sacrament?) Objection #2 says "Only the flesh and blood of Christ are contained in this sacrament. But there are many other parts of Christ's body, for instance, the nerves, bones, and such like." Thomas' response: "It is absolutely necessary to confess according to Catholic faith that the entire Christ is in this sacrament."
"Material food first changes into the one who eats it, and then, as a consequence, restores to him lost strength and increases his vitality. Spiritual food, on the other hand, changes the person who eats it into itself. Thus the effect proper to this Sacrament is the conversion of a man into Christ, so that he may no longer live, but Christ lives in him; consequently, it has the double effect of restoring the spiritual strength he had lost by his sins and defects, and of increasing the strength of his virtues." St. Thomas, Commentary on Book IV of the Sentences, d. 12, q. 2, a. 11.