To the explanations of the great but unregarded Christine Mohrmann, I will add the careful elucidations of another brilliant but unregarded liturgist of the mid-century, Craddock Ratcliff, an Anglican.
Ratcliff demonstrates the great antiquity of the Roman Institution Narratives by remarking that they antedate the Vulgate. " ... it is not unreasonable to infer that the Institution Narrative of the Roman Canon belongs to a liturgical tradition for which the careful preservation of the scriptural form and character of the narrative was held to be vital ... for the sacrificium novum to be right, valid, acceptable and effectual, it must be celebrated, as was the sacrificium vetus, in the correct manner prescribed by the Lord ... Any deviation from the procedure here enjoined vitiates the rite, so that there can be no assurance that the sacrifice of the Passion is offered." Ratcliff quotes S Cyprian: "[non] sacrificium dominicum legitima sanctificatione celebrari, nisi oblatio et sacrificium nostrum responderit passioni ... quia ... passio est ... domini sacrificium quod offerimus, nihil aliud quam quod ille fecit facere debemus". Hence the function of the Narrative in the Canon is not merely to revive the memory of a significant historical event, or to provide a rationale for the celebration of the Eucharist, as the Greek Narratives do; its function is rather to make the significant historic event continuously present and operative. By means of the Narrative, theefore, the Church's actio in the Eucharist is identifierd with, and becomes, the actio of Christ at the Institution".