Among the many treatises on the Mass written from the times of the Fathers through the High Middle Ages, the most concise is that of the teacher and later monk at Cluny, Alger of Liege (d. 1131).  The whole of this work of his, is translated and presented below:

"The solemn celebration of the Mass was instituted in order to express the memory of Christ coming into the world, and to refashion the mystery of his Passion.  Thus, the parts of the Mass that are first carried out, from the Introit to the Secrets, which the priest imparts, speaking them to himself, represent the coming of The Lord and the times of the preaching of the Gospel.  Then the Secrets, which are imparted over the offerings, pour forth the prayer which the Savior made three times before his Passion: 'Father, if it may be done, let this chalice pass from me' (Matthew 26, 39).   It is written in that passage that The Lord sweat blood and prayed for a long time in his agony.  The priest, as sinner, should plainly show that he offers the Sacrifice of prayer to God with such contrition of heart and solicitude of spirit as when the Priest of Justice offered himself, sending forth his prayers to the Father with the pouring out of his Blood, and plainly showing to us the power of his compassion.  The lawful Priest, who never enters the holy of holies without his Blood, does this, for according to the Apostle, there is no forgiveness of sins without the pouring out of blood.  Afterwards, our priest greets the people with a loud voice.  He exhorts them to lift up their hearts to The Lord and to give thanks, expressing what The Lord said, turning to his disciples: 'Watch and pray that you may not enter into temptation' (Matthew 26, 41).  Next follows that heavenly hymn, 'Holy, holy, holy, Lord God of hosts', which the winged spirits are said to have sung in the Old Testament.  This shows that the angelic spirits are present at the hour of Sacrifice, showing due reverence for the divine mysteries.

"Next, our priest begins the words of the Canon, and as if entering the holy of holies, first pours forth a general prayer on behalf of the whole Church, making the sign of the Cross, pouring the Blood of Christ over this offering, for as often as he imparts the sign of the Cross to the heavenly offering, he sprinkles the Blood of Christ over the offering.  Those things which he offers he first calls 'gifts', because they are given to us by the Father of lights.  Second, he calls them 'offerings', because we offer the one who is praying through them.  Third, he calls them 'sacrifices', because we are washed from our sins through them, and when we have been reconciled, we have peace with God.  The second prayer in the canon is made on behalf of those who either have offered the bread for the sacrifice, or they are present at the sacred mysteries.  The priest implores the assistance of the saints in the third prayer, and dutifully imposes the incense of prayer in order to, as it were, overshadow the offering of the heavenly Priest with a cloud of compunction.  Next, the priest prayers for this sacrifice to be made blessed so that we may be filled with every heavenly blessing through it.  He prays for it to be approved so that by its efficacious power we may be approved in the book of the living.  And he prays for it to be ratified so that, rooted or established in faith through its power, in no wise may we be snatched away from the love of God.

"Next, the priest recounts the Lord's words, which Christ said, by whose power and grace the bread is changed into his Body, and wine into his Blood, and which the Catholic Faith confesses.  But why is the chalice of his Blood called a new and eternal covenant?  Because the forgiveness of our sins and the receiving of an eternal kingdom is promised to us through faith in the Blood of Jesus Christ.  It is said to be both new and eternal in that we are made new when he purges us from our sins, and he grants us a kingdom when he changes that which is lifted up in offering from that which is corrupt and unstable into eternal and incorruptible.  Further on, the Christian Faith holds that this is a mystery of faith.  The Faith confesses the truth that this mystery is both and at the same time a sacrament, and the matter of the sacrament, under the appearances of bread and wine.  Indeed, the exterior color and taste of the bread and wine are present, but the interior substance of the Body and Blood is afforded to the eyes of faith alone.  The priest is then able to call the visible elements of the Sacrifice pure because he has purged it from all that is vile.  He calls them holy because they confer the grace of the virtues.  He calls them immaculate because they grant the glory of incorruption to the just.  The priest prays for this Sacrifice to be made acceptable just as were the gifts of Abel, whose innocence signifies the innocence and life of Christ; as was the sacrifice of Abraham, because it signifies the sanctity of the Lord's Passion; and as the sacrifice of Melchizedek, which prefigured the daily Sacrifice of the Church in bread and wine.  The priest then prays The Lord that he command that this Sacrifice be raised up by the hands of a holy angel unto his altar on high, that the Sacrament may be manifest at that time, the bread united to the Body of The Lord, and joined in the unity of the same substance.  He next kisses the altar in order to show that he wishes to be made a sharer in this Sacrament, and he strengthens himself with the sign of the Cross in order to prepare himself for the reception of this mystery.  He follows this by praying for the dead and also for himself, and, at the end, he includes the names of the saints, as though he were another Aaron, speaking on behalf of the twelve Patriarchs whose names were inscribed on his breast (cf. Exodus 28, 29).

"After this, he prays, 'Through whom you create all these good things, O Lord', by bringing forth the substance of his Flesh from the bread; 'You make holy', by the gathering together the abundance of spiritual virtues; 'You bless', by granting the efficacy of all graces.  Subsequently, he removes the corporal from the chalice and makes the sign of the Cross over it with the sanctified Bread, showing the Crucifixion of Christ.  Then he places the Host on the altar again, he covers the chalice with the corporal to signify the three days The Lord spent in the tomb.  He tells us that this Sacrament of power has been completed when he prays, 'Through him', who gives richly to all and does not reproach anyone; 'With him', who rules and governs all things; 'In him', in whom we live, and move, and have our being.  The Lord's Prayer follows, concluding with the words, 'Deliver us from evil'.  At the end of this prayer, the priest divides the Bread of The Lord into three parts.  The first he puts into the chalice.  The second part he himself consumes.  The third part he offers to one who receiving Communion.  The fraction of the Bread signifies the Passion of Christ during which the Temple of his Body was broken and destroyed, and three days afterwards it was awakened by the divine power.  The chalice signifies the tomb of The Lord, in which Christ was swallowed up by death, so that he lay in the heart of the earth for three days just as Jonah was swallowed up by the belly of the fish for three days.  The priest prays for those who have already gone out from their bodies With the first part of the Host, which is mixed with the Blood, the priest prays for those who have already gone out from their bodies, but bound by the sins of their bodies, they have in themselves that which must be tried and purged by the pains of purgatory.  With the middle part he prays for those who still live in the flesh, and, as though in a middle position, they are able to decide with their own free will whether they wish to go to the right or to the left hand of The Lord.  With the third part he gives thanks for the saints established in glory for whom there is no need for prayer.  In this way, the Passion of Christ profits everyone: the dead, for forgiveness; the living, for grace; and the saints, for glory.  Likewise, the Sacrament brings good to each person: it purges the dead, justifies the living, and crowns the saints.  Through him who lives and reigns for ever and ever.  Amen."