Objection 1. It seems that Christ's body is movably in this sacrament, because the Philosopher says (Topic. ii) that "when we are moved, the things within us are moved": and this is true even of the soul's spiritual substance. "But Christ is in this sacrament," as shown above (74, 1). Therefore He is moved when it is moved.
Objection 2. Further, the truth ought to correspond with the figure. But, according to the commandment (Exodus 12:10), concerning the Paschal Lamb, a figure of this sacrament, "there remained nothing until the morning." Neither, therefore, if this sacrament be reserved until morning, will Christ's body be there; and so it is not immovably in this sacrament.
Objection 3. Further, if Christ's body were to remain under this sacrament even until the morrow, for the same reason it will remain there during all coming time; for it cannot be said that it ceases to be there when the species pass, because the existence of Christ's body is not dependent on those species. Yet Christ does not remain in this sacrament for all coming time. It seems, then, that straightway on the morrow, or after a short time, He ceases to be under this sacrament. And so it seems that Christ is in this sacrament movably.
On the contrary, it is impossible for the same thing to be in motion and at rest, else contradictories would be verified of the same subject. But Christ's body is at rest in heaven. Therefore it is not movably in this sacrament.
I answer that, When any thing is one, as to subject, and manifold in being, there is nothing to hinder it from being moved in one respect, and yet to remain at rest in another just as it is one thing for a body to be white, and another thing, to be large; hence it can be moved as to its whiteness, and yet continue unmoved as to its magnitude. But in Christ, being in Himself and being under the sacrament are not the same thing, because when we say that He is under this sacrament, we express a kind of relationship to this sacrament. According to this being, then, Christ is not moved locally of Himself, but only accidentally, because Christ is not in this sacrament as in a place, as stated above (Article 5). But what is not in a place, is not moved of itself locally, but only according to the motion of the subject in which it is.
In the same way neither is it moved of itself according to the being which it has in this sacrament, by any other change whatever, as for instance, that it ceases to be under this sacrament: because whatever possesses unfailing existence of itself, cannot be the principle of failing; but when something else fails, then it ceases to be in it; just as God, Whose existence is unfailing and immortal, ceases to be in some corruptible creature because such corruptible creature ceases to exist. And in this way, since Christ has unfailing and incorruptible being, He ceases to be under this sacrament, not because He ceases to be, nor yet by local movement of His own, as is clear from what has been said, but only by the fact that the sacramental species cease to exist.
Hence it is clear that Christ, strictly speaking is immovably in this sacrament.
Reply to Objection 1. This argument deals with accidental movement, whereby things within us are moved together with us. But with things which can of themselves be in a place, like bodies, it is otherwise than with things which cannot of themselves be in a place, such as forms and spiritual substances. And to this mode can be reduced what we say of Christ, being moved accidentally, according to the existence which He has in this sacrament, in which He is not present as in a place.
Reply to Objection 2. It was this argument which seems to have convinced those who held that Christ's body does not remain under this sacrament if it be reserved until the morrow. It is against these that Cyril says (Ep. lxxxiii): "Some are so foolish as to say that the mystical blessing departs from the sacrament, if any of its fragments remain until the next day: for Christ's consecrated body is not changed, and the power of the blessing, and the life-giving grace is perpetually in it." Thus are all other consecrations irremovable so long as the consecrated things endure; on which account they are not repeated. And although the truth corresponds with the figure, still the figure cannot equal it.
Reply to Objection 3. The body of Christ remains in this sacrament not only until the morrow, but also in the future, so long as the sacramental species remain: and when they cease, Christ's body ceases to be under them, not because it depends on them, but because the relationship of Christ's body to those species is taken away, in the same way as God ceases to be the Lord of a creature which ceases to exist.
Source: Summa Theologica (New Advent)