Objection 1. It seems that the substance of the bread and wine does remain in this sacrament after the consecration: because Damascene says (De Fide Orth. iv): "Since it is customary for men to eat bread and drink wine, God has wedded his Godhead to them, and made them His body and blood": and further on: "The bread of communication is not simple bread, but is united to the Godhead." But wedding together belongs to things actually existing. Therefore the bread and wine are at the same time, in this sacrament, with the body and the blood of Christ.
Objection 2. Further, there ought to be conformity between the sacraments. But in the other sacraments the substance of the matter remains, like the substance of water in Baptism, and the substance of chrism in Confirmation. Therefore the substance of the bread and wine remains also in this sacrament.
Objection 3. Further, bread and wine are made use of in this sacrament, inasmuch as they denote ecclesiastical unity, as "one bread is made from many grains and wine from many grapes," as Augustine says in his book on the Creed (Tract. xxvi in Joan.). But this belongs to the substance of bread and wine. Therefore, the substance of the bread and wine remains in this sacrament.
On the contrary, Ambrose says (De Sacram. iv): "Although the figure of the bread and wine be seen, still, after the Consecration, they are to be believed to be nothing else than the body end blood of Christ."
I answer that, Some have held that the substance of the bread and wine remains in this sacrament after the consecration. But this opinion cannot stand: first of all, because by such an opinion the truth of this sacrament is destroyed, to which it belongs that Christ's true body exists in this sacrament; which indeed was not there before the consecration. Now a thing cannot be in any place, where it was not previously, except by change of place, or by the conversion of another thing into itself; just as fire begins anew to be in some house, either because it is carried thither, or because it is generated there. Now it is evident that Christ's body does not begin to be present in this sacrament by local motion. First of all, because it would follow that it would cease to be in heaven: for what is moved locally does not come anew to some place unless it quit the former one. Secondly, because every body moved locally passes through all intermediary spaces, which cannot be said here. Thirdly, because it is not possible for one movement of the same body moved locally to be terminated in different places at the one time, whereas the body of Christ under this sacrament begins at the one time to be in several places. And consequently it remains that Christ's body cannot begin to be anew in this sacrament except by change of the substance of bread into itself. But what is changed into another thing, no longer remains after such change. Hence the conclusion is that, saving the truth of this sacrament, the substance of the bread cannot remain after the consecration.
Secondly, because this position is contrary to the form of this sacrament, in which it is said: "This is My body," which would not be true if the substance of the bread were to remain there; for the substance of bread never is the body of Christ. Rather should one say in that case: "Here is My body."
Thirdly, because it would be opposed to the veneration of this sacrament, if any substance were there, which could not be adored with adoration of latria.
Fourthly, because it is contrary to the rite of the Church, according to which it is not lawful to take the body of Christ after bodily food, while it is nevertheless lawful to take one consecrated host after another. Hence this opinion is to be avoided as heretical.
Reply to Objection 1. God "wedded His Godhead," i.e. His Divine power, to the bread and wine, not that these may remain in this sacrament, but in order that He may make from them His body and blood.
Reply to Objection 2. Christ is not really present in the other sacraments, as in this; and therefore the substance of the matter remains in the other sacraments, but not in this.
Reply to Objection 3. The species which remain in this sacrament, as shall be said later (5), suffice for its signification; because the nature of the substance is known by its accidents.
Source: Summa Theologica (New Advent)