Last year, when I had received my Volume II of the 1909 Breviarium S.O.P., among the many treasures that I discovered between its covers, were the "Modus Terminandi Horas"...a set of versicles, responses, antiphons and prayers that, according to the rubrics, were prayed at the end of each hour, Compline being the only exception.Recently, while flipping through Fr. William Bonniwell's "History of the Dominican Liturgy" (1944) again, I stumbled across a passage which indicates the origin of these prayers...at least most of them.
While writing about the infamous dispute between Pope John XXII and orthodox theologians regarding the particular judgement, he notes how a Dominican master of theology at Avignon denounced the Popes opinion, and was cast into a dungeon at the Popes command.
Fr. Bonniwell continues: "This ungenerous conduct on the part of the Pope estranged the Dominicans who, as we shall presently see, had lately suffered greatly in his defense. As relations grew more and more bitter between the Pope and the Order, the chapter of 1334 directed the friars to recommence the litany and to add a new prayer to the Divine Office:
Because our Order places its trust in a most special way in the protection of the glorious Virgin, in this regard following the example of our holy fathers, we desire and ordain for the peace and safety of our Order that, whenever the Fidelium is said in choir at the end of the hours, immediately after it the friars are to recite, while kneeling, the Salve Regina, with the versicle Ora pro nobis, and the versicle Esto nobis Domine, with the prayer, Protege Domine and Ecclesiae tuae...This does not apply to compline, when the Salve is sung" (Acta Cap. Gen., II, 223.)
Now if you look at the prayers at the end of the Psalter in the 1909 Breviarium, you see the Salve Regina, the versicle Ora pro nobis (though it is addressed to St. Dominic, not the Blessed Mother), and the prayers, Protege Domine and Ecclesiae tuae.. Missing from the list from the Chapter of 1334 is the versicle Esto nobis Domine. Numerous other versicles were added, I suppose, over time.So it appears that the inclusion of these prayers after the hours dates from the early 14 Century, and the Order's conflict with an Avignon Pope and his heterodox opinion. And, as it has done for 800 years now, when the Order finds itself under siege, whether it be by a Pope or the Devil, it always turns with love and confidence to our Blessed Lady, Patroness of the Order of Preachers.