The Feast of Corpus Christi, although a relatively late addition to the Roman temporale, is certainly one of the most interesting feasts from a historian's point of view. Among the intriguing problems are the question of St. Thomas Aquinas' authorship of the Office and the Mass; the arrangement of the Office in the years before the reforms of Pius V, regardless of who may originally have composed it; the use of borrowed material from other feasts; and the nature of the chants themselves. Though very little attention has been paid to the Office for the Feast of Corpus Christi by musicologists, who have concentrated instead on the music for the Corpus Christi processions and plays,' liturgical scholars have debated for the last seventy years the arrangement of this Office and, to a lesser degree, some of the other problems just outlined.2 Their study has centered on the interpretation of the relatively few liturgical books of the late 13th and 14th centuries that contain this Office. To this limited number can now be added