St. Thomas Aquinas
“After what we have said above, 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 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.”
Source: St. Thomas Aquinas, Summa Theologica