Chapter 18 — Part II: The Sacraments — The Holy Eucharist
Section 89. On the other hand, some things are contained in the Sacrament because they are united to those which are expressed in the form. For instance, the words This is my body, which comprise the form used to consecrate the bread, signify the body of the Lord, and hence the body itself of Christ the Lord is contained in the Eucharist by virtue of the Sacrament. Since, however, to Christ's body are united His blood, His soul, and His Divinity, all of these also must be found to coexist in the Sacrament; not, however, by virtue of the consecration, but by virtue of the union that subsists between them and His body. All these are said to be in the Eucharist by virtue of concomitance. Hence it is clear that Christ, whole and entire, is contained in the Sacrament; for when two things are actually united, where one is, the other must also be.