Objection 1. It seems that the whole Christ is not contained under this sacrament, because Christ begins to be in this sacrament by conversion of the bread and wine. But it is evident that the bread and wine cannot be changed either into the Godhead or into the soul of Christ. Since therefore Christ exists in three substances, namely, the Godhead, soul and body, as shown above (2, 5; 5, 1,3), it seems that the entire Christ is not under this sacrament.
Objection 2. Further, Christ is in this sacrament, forasmuch as it is ordained to the refection of the faithful, which consists in food and drink, as stated above (Question 74, Article 1). But our Lord said (John 6:56): "My flesh is meat indeed, and My blood is drink indeed." Therefore, 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. Therefore the entire Christ is not contained under this sacrament.
Objection 3. Further, a body of greater quantity cannot be contained under the measure of a lesser. But the measure of the bread and wine is much smaller than the measure of Christ's body. Therefore it is impossible that the entire Christ be contained under this sacrament.
On the contrary, Ambrose says (De Officiis): "Christ is in this sacrament."
I answer that, It is absolutely necessary to confess according to Catholic faith that the entire Christ is in this sacrament. Yet we must know that there is something of Christ in this sacrament in a twofold manner: first, as it were, by the power of the sacrament; secondly, from natural concomitance. By the power of the sacrament, there is under the species of this sacrament that into which the pre-existing substance of the bread and wine is changed, as expressed by the words of the form, which are effective in this as in the other sacraments; for instance, by the words: "This is My body," or, "This is My blood." But from natural concomitance there is also in this sacrament that which is really united with that thing wherein the aforesaid conversion is terminated. For if any two things be really united, then wherever the one is really, there must the other also be: since things really united together are only distinguished by an operation of the mind.
Reply to Objection 1. Because the change of the bread and wine is not terminated at the Godhead or the soul of Christ, it follows as a consequence that the Godhead or the soul of Christ is in this sacrament not by the power of the sacrament, but from real concomitance. For since the Godhead never set aside the assumed body, wherever the body of Christ is, there, of necessity, must the Godhead be; and therefore it is necessary for the Godhead to be in this sacrament concomitantly with His body. Hence we read in the profession of faith at Ephesus (P. I., chap. xxvi): "We are made partakers of the body and blood of Christ, not as taking common flesh, nor as of a holy man united to the Word in dignity, but the truly life-giving flesh of the Word Himself."
On the other hand, His soul was truly separated from His body, as stated above (Question 50, Article 5). And therefore had this sacrament been celebrated during those three days when He was dead, the soul of Christ would not have been there, neither by the power of the sacrament, nor from real concomitance. But since "Christ rising from the dead dieth now no more" (Romans 6:9), His soul is always really united with His body. And therefore in this sacrament the body indeed of Christ is present by the power of the sacrament, but His soul from real concomitance.
Reply to Objection 2. By the power of the sacrament there is contained under it, as to the species of the bread, not only the flesh, but the entire body of Christ, that is, the bones the nerves, and the like. And this is apparent from the form of this sacrament, wherein it is not said: "This is My flesh," but "This is My body." Accordingly, when our Lord said (John 6:56): "My flesh is meat indeed," there the word flesh is put for the entire body, because according to human custom it seems to be more adapted for eating, as men commonly are fed on the flesh of animals, but not on the bones or the like.
Reply to Objection 3. As has been already stated (75, 5), after the consecration of the bread into the body of Christ, or of the wine into His blood, the accidents of both remain. From which it is evident that the dimensions of the bread or wine are not changed into the dimensions of the body of Christ, but substance into substance. And so the substance of Christ's body or blood is under this sacrament by the power of the sacrament, but not the dimensions of Christ's body or blood. Hence it is clear that the body of Christ is in this sacrament "by way of substance," and not by way of quantity. But the proper totality of substance is contained indifferently in a small or large quantity; as the whole nature of air in a great or small amount of air, and the whole nature of a man in a big or small individual. Wherefore, after the consecration, the whole substance of Christ's body and blood is contained in this sacrament, just as the whole substance of the bread and wine was contained there before the consecration.
Source: Summa Theologica (New Advent)