Objection 1. It seems that the whole Christ is not contained under both species of this sacrament. For this sacrament is ordained for the salvation of the faithful, not by virtue of the species, but by virtue of what is contained under the species, because the species were there even before the consecration, from which comes the power of this sacrament. If nothing, then, be contained under one species, but what is contained under the other, and if the whole Christ be contained under both, it seems that one of them is superfluous in this sacrament.
Objection 2. Further, it was stated above (1, ad 1) that all the other parts of the body, such as the bones, nerves, and the like, are comprised under the name of flesh. But the blood is one of the parts of the human body, as Aristotle proves (De Anima Histor. i). If, then, Christ's blood be contained under the species of bread, just as the other parts of the body are contained there, the blood ought not to be consecrated apart, just as no other part of the body is consecrated separately.
Objection 3. Further, what is once "in being" cannot be again "in becoming." But Christ's body has already begun to be in this sacrament by the consecration of the bread. Therefore, it cannot begin again to be there by the consecration of the wine; and so Christ's body will not be contained under the species of the wine, and accordingly neither the entire Christ. Therefore the whole Christ is not contained under each species.
On the contrary, The gloss on 1 Corinthians 11:25, commenting on the word "Chalice," says that "under each species," namely, of the bread and wine, "the same is received"; and thus it seems that Christ is entire under each species.
I answer that, After what we have said above (Article 1), it must be held most certainly that the whole Christ is under each sacramental species yet not alike in each. For the body of Christ is indeed present under the species of bread by the power of the sacrament, while the blood is there from real concomitance, as stated above (1, ad 1) in regard to the soul and Godhead of Christ; and under the species of wine the blood is present by the power of the sacrament, and His body by real concomitance, as is also His soul and Godhead: because now Christ's blood is not separated from His body, as it was at the time of His Passion and death. Hence if this sacrament had been celebrated then, the body of Christ would have been under the species of the bread, but without the blood; and, under the species of the wine, the blood would have been present without the body, as it was then, in fact.
Reply to Objection 1. Although the whole Christ is under each species, yet it is so not without purpose. For in the first place this serves to represent Christ's Passion, in which the blood was separated from the body; hence in the form for the consecration of the blood mention is made of its shedding. Secondly, it is in keeping with the use of this sacrament, that Christ's body be shown apart to the faithful as food, and the blood as drink. Thirdly, it is in keeping with its effect, in which sense it was stated above (Question 74, Article 1) that "the body is offered for the salvation of the body, and the blood for the salvation of the soul."
Reply to Objection 2. In Christ's Passion, of which this is the memorial, the other parts of the body were not separated from one another, as the blood was, but the body remained entire, according to Exodus 12:46: "You shall not break a bone thereof." And therefore in this sacrament the blood is consecrated apart from the body, but no other part is consecrated separately from the rest.
Reply to Objection 3. As stated above, the body of Christ is not under the species of wine by the power of the sacrament, but by real concomitance: and therefore by the consecration of the wine the body of Christ is not there of itself, but concomitantly.
Source: Summa Theologica (New Advent)