Chapter 18 — Part II: The Sacraments — The Holy Eucharist
Section 50. That these words constitute the form is easily proved from reason also. The form is that which signifies what is accomplished in this Sacrament; but as the preceding words signify and declare what takes place in the Eucharist, that is, the conversion of the bread into the true body of our Lord, it therefore follows that these very words constitute the form. In this sense may be understood the words of the Evangelist: He blessed; for they seem equivalent to this: Taking bread, he blessed it, saying: "This is my body".