Even in the West the Roman law never wholly died out. The Roman subjects of the 'barbarian' rulers in southern France, Spain, and Italy continued to live by the 'vulgar law' to be found in compilations such as the lex Romana Visigothorum. In Italy, it is true, Justinian's codification had been promulgated in 554, and some fragmentary knowledge of the Institutes, Code, and Novels survived, but the Digest was entirely forgotten. And in northern Europe almost all trace of Roman law disappeared. (Zimmermann, 1996)
The great influence of Roman law derives, however, not from this attenuated survival, but from the revival which began, as part of a wider renaissance of learning, at the end of the eleventh century. The Digest was rediscovered and for the first time thoroughly mastered. The first and greatest figure in this revival was Irnerius (c. 1055-c. 1130) who taught at Bologna. He and his successors at Bologna set themselves the task of elucidating, harmonizing, and expounding the Corpus Iuris text by text.
(Thomas, 1976) The main literary form which this work took was the note or gloss written in the margin or between the lines of the text to explain its meaning and to provide the cross-references and reconciliations without which the work was unusable. For this reason Irnerius and his successors are called the Glossators, but their writings took also other forms, both systematic and controversial. Enthusiasm for the new learning was immense. Students came to Bologna from all Western Europe, and by the middle of the twelfth century their number had reached 10,000.
Moreover, from Bologna the study of Roman law spread to other nascent universities in Italy and far beyond. In the later twelfth century, for example, Vacarius was teaching it in England. The main impulse continued, however, to come from Italy, and after little more than a century the work of the Glossators was done. It was summed up and completed by Accursius (d. 1260), who combined all the glosses into one great gloss. This glossa ordinaria thereafter held the field alone, both in the manuscript versions of the Corpus Iuris and in the early printed editions.
This revival of Roman law was an academic revival, both in the sense that it originated in the universities and in the sense that it was unconcerned with the law which was applied in the courts. In the former sense its academic character was never lost—and has to a large extent been transmitted to the modern civil law—but the practical law could not long remain uninfluenced by the ferment in the universities. And there was a corresponding change in the direction of academic interest.
The successors of Accursius turned to the practical application of Roman law to the problems of their own time. For this purpose the method of the gloss was inadequate, and its place was taken by more systematic and extensive commentaries, the authors of which are therefore commonly called the Commentators (or Post-glossators). By applying, as occasion demanded, a restrictive or liberal interpretation, and by the use of fine distinctions, they adapted the ancient law to medieval needs.
The material on which they worked was more often the gloss than the texts themselves, and the law which emerged was thus Roman law at third hand, but it was practical law and in this lay its strength. The greatest of the Commentators were Bartolus (1314-57) and his pupil, Baldus (1327-1400), whose authority was such that only the foolhardy would argue against it. Nemo iurista nisi Bartolista ran the maxim.