The Structure of the Mass, Its Elements and Its Parts
THE GENERAL STRUCTURE OF THE MASS
27. At Mass that is, the Lord's Supper the People of God is called together, with a priest presiding and acting in the person of Christ, to celebrate the memorial of the Lord, the Eucharistic Sacrifice.[37] For this reason Christ's promise applies in an outstanding way to such a local gathering of the holy Church: "Where two or three are gathered in my name, there am I in their midst" (Mt 18:20). For in the celebration of Mass, in which the Sacrifice of the Cross is perpetuated,[38] Christ is really present in the very liturgical assembly gathered in his name, in the person of the minister, in his word, and indeed substantially and continuously under the Eucharistic species.[39]
28. The Mass is made up, as it were, of two parts: the Liturgy of the Word and the Liturgy of the Eucharist. These, however, are so closely interconnected that they form but one single act of worship.[40] For in the Mass the table both of God's word and of Christ's Body is prepared, from which the faithful may be instructed and refreshed.[41] There are also certain rites that open and conclude the celebration.
THE DIFFERENT ELEMENTS OF THE MASS
The Prayers and Other Parts Pertaining to the Priest
30. Among the parts assigned to the priest, the foremost is the Eucharistic Prayer, which is the high point of the entire celebration. Next are the orations: that is to say, the collect, the prayer over the offerings, and the prayer after Communion. These prayers are addressed to God in the name of the entire holy people and all present, by the priest who presides over the assembly in the person of Christ.[43] It is with good reason, therefore, that they are called the "presidential prayers."
31. It is also up to the priest, in the exercise of his office of presiding over the gathered assembly, to offer certain explanations that are foreseen in the rite itself. Where it is indicated in the rubrics, the celebrant is permitted to adapt them somewhat in order that they respond to the understanding of those participating. However, he should always take care to keep to the sense of the text given in the Missal and to express it succinctly. The presiding priest is also to direct the word of God and to impart the final blessing. In addition, he may give the faithful a very brief introduction to the Mass of the day (after the initial Greeting and before the Act of Penitence), to the Liturgy of the Word (before the readings), and to the Eucharistic Prayer (before the Preface), though never during the Eucharistic Prayer itself; he may also make concluding comments to the entire sacred action before the dismissal.
32. The nature of the "presidential" texts demands that they be spoken in a loud and clear voice and that everyone listen with attention.[44] Thus, while the priest is speaking these texts, there should be no other prayers or singing, and the organ or other musical instruments should be silent.
33. The priest, in fact, as the one who presides, prays in the name of the Church and of the assembled community; but at times he prays only in his own name, asking that he may exercise his ministry with greater attention and devotion. Prayers of this kind, which occur before the reading of the Gospel, at the Preparation of the Gifts, and also before and after the Communion of the priest, are said quietly.
From: GENERAL INSTRUCTION OF THE ROMAN MISSAL