Objection 1. It seems that this is not the form of this sacrament: "This is My body," and, "This is the chalice of My blood." Because those words seem to belong to the form of this sacrament, wherewith Christ consecrated His body and blood. But Christ first blessed the bread which He took, and said afterwards: "Take ye and eat; this is My body" (Matthew 26:26). Therefore the whole of this seems to belong to the form of this sacrament: and the same reason holds good of the words which go with the consecration of the blood.
Objection 2. Further, Eusebius Emissenus (Pseudo-Hieron: Ep. xxix; Pseudo-Isid.: Hom. iv) says: "The invisible Priest changes visible creatures into His own body, saying: 'Take ye and eat; this is My body.'" Therefore, the whole of this seems to belong to the form of this sacrament: and the same hold good of the works appertaining to the blood.
Objection 3. Further, in the form of Baptism both the minister and his act are expressed, when it is said, "I baptize thee." But in the words set forth above there is no mention made either of the minister or of his act. Therefore the form of the sacrament is not a suitable one.
Objection 4. Further, the form of the sacrament suffices for its perfection; hence the sacrament of Baptism can be performed sometimes by pronouncing the words of the form only, omitting all the others. Therefore, if the aforesaid words be the form of this sacrament, it would seem as if this sacrament could be performed sometimes by uttering those words alone, while leaving out all the others which are said in the mass; yet this seems to be false, because, were the other words to be passed over, the said words would be taken as spoken in the person of the priest saying them, whereas the bread and wine are not changed into his body and blood. Consequently, the aforesaid words are not the form of this sacrament.
On the contrary, Ambrose says (De Sacram. iv): "The consecration is accomplished by the words and expressions of the Lord Jesus. Because, by all the other words spoken, praise is rendered to God, prayer is put up for the people, for kings, and others; but when the time comes for perfecting the sacrament, the priest uses no longer his own words, but the words of Christ. Therefore, it is Christ's words that perfect this sacrament."
I answer that, This sacrament differs from the other sacraments in two respects. First of all, in this, that this sacrament is accomplished by the consecration of the matter, while the rest are perfected in the use of the consecrated matter. Secondly, because in the other sacraments the consecration of the matter consists only in a blessing, from which the matter consecrated derives instrumentally a spiritual power, which through the priest who is an animated instrument, can pass on to inanimate instruments. But in this sacrament the consecration of the matter consists in the miraculous change of the substance, which can only be done by God; hence the minister in performing this sacrament has no other act save the pronouncing of the words. And because the form should suit the thing, therefore the form of this sacrament differs from the forms of the other sacraments in two respects. First, because the form of the other sacraments implies the use of the matter, as for instance, baptizing, or signing; but the form of this sacrament implies merely the consecration of the matter, which consists in transubstantiation, as when it is said, "This is My body," or, "This is the chalice of My blood." Secondly, because the forms of the other sacraments are pronounced in the person of the minister, whether by way of exercising an act, as when it is said, "I baptize thee," or "I confirm thee," etc.; or by way of command, as when it is said in the sacrament of order, "Take the power," etc.; or by way of entreaty, as when in the sacrament of Extreme Unction it is said, "By this anointing and our intercession," etc. But the form of this sacrament is pronounced as if Christ were speaking in person, so that it is given to be understood that the minister does nothing in perfecting this sacrament, except to pronounce the words of Christ.
Reply to Objection 1. There are many opinions on this matter. Some have said that Christ, Who had power of excellence in the sacraments, performed this sacrament without using any form of words, and that afterwards He pronounced the words under which others were to consecrate thereafter. And the words of Pope Innocent III seem to convey the same sense (De Sacr. Alt. Myst. iv), where he says: "In good sooth it can be said that Christ accomplished this sacrament by His Divine power, and subsequently expressed the form under which those who came after were to consecrate." But in opposition to this view are the words of the Gospel in which it is said that Christ "blessed," and this blessing was effected by certain words. Accordingly those words of Innocent are to be considered as expressing an opinion, rather than determining the point.
Others, again, have said that the blessing was effected by other words not known to us. But this statement cannot stand, because the blessing of the consecration is now performed by reciting the things which were then accomplished; hence, if the consecration was not performed then by these words, neither would it be now.
Accordingly, others have maintained that this blessing was effected by the same words as are used now; but that Christ spoke them twice, at first secretly, in order to consecrate, and afterwards openly, to instruct others. But even this will not hold good, because the priest in consecrating uses these words, not as spoken in secret, but as openly pronounced. Accordingly, since these words have no power except from Christ pronouncing them, it seems that Christ also consecrated by pronouncing them openly.
And therefore others said that the Evangelists did not always follow the precise order in their narrative as that in which things actually happened, as is seen from Augustine (De Consens. Evang. ii). Hence it is to be understood that the order of what took place can be expressed thus: "Taking the bread He blessed it, saying: This is My body, and then He broke it, and gave it to His disciples." But the same sense can be had even without changing the words of the Gospel; because the participle "saying" implies sequence of the words uttered with what goes before. And it is not necessary for the sequence to be understood only with respect to the last word spoken, as if Christ had just then pronounced those words, when He gave it to His disciples; but the sequence can be understood with regard to all that had gone before; so that the sense is: "While He was blessing, and breaking, and giving it to His disciples, He spoke the words, 'Take ye,'" etc.
Reply to Objection 2. In these words, "Take ye and eat," the use of the consecrated matter is indicated, which is not of the necessity of this sacrament, as stated above (Question 74, Article 7). And therefore not even these words belong to the substance of the form. Nevertheless, because the use of the consecrated matter belongs to a certain perfection of the sacrament, in the same way as operation is not the first but the second perfection of a thing, consequently, the whole perfection of this sacrament is expressed by all those words: and it was in this way that Eusebius understood that the sacrament was accomplished by those words, as to its first and second perfection.
Reply to Objection 3. In the sacrament of Baptism the minister exercises an act regarding the use of the matter, which is of the essence of the sacrament: such is not the case in this sacrament; hence there is no parallel.
Reply to Objection 4. Some have contended that this sacrament cannot be accomplished by uttering the aforesaid words, while leaving out the rest, especially the words in the Canon of the Mass. But that this is false can be seen both from Ambrose's words quoted above, as well as from the fact that the Canon of the Mass is not the same in all places or times, but various portions have been introduced by various people.
Accordingly it must be held that if the priest were to pronounce only the aforesaid words with the intention of consecrating this sacrament, this sacrament would be valid because the intention would cause these words to be understood as spoken in the person of Christ, even though the words were pronounced without those that precede. The priest, however, would sin gravely in consecrating the sacrament thus, as he would not be observing the rite of the Church. Nor does the comparison with Baptism prove anything; for it is a sacrament of necessity: whereas the lack of this sacrament can be supplied by the spiritual partaking thereof, as Augustine says (cf. 73, 3, ad 1).
Source: Summa Theologica (New Advent)