During this golden age of Roman jurisprudence many commentaries were written on the Twelve Tables, the Perpetual Edict, the Laws of the People, and the Decrees of the senate, as well as a vast mass of treatises on every department of the law, most of which have perished.  The Institutes of Gaius, already mentioned, are the most valuable that remain, and have thrown great light on some important branches previously involved in obscurity.  Their use in explaining the Institutes of Justinian is spoken of very highly by Mackenzie, since the latter are mainly founded on the long-lost work of Gaius.  The great lawyers who flourished from Trajan to Alexander Severus, like Gaius, Ulpian, Paulus, Papinian, and Modestinus, had no successors who can be compared with them, and their works became standard authorities in the courts of law.

After the death of Alexander Severus, 235 A.D., no great accession was made to Roman law until Theodosius II., 438 A.D., caused the constitutions, from Constantine to his own time, to be collected and arranged in sixteen books.  This was called the Theodosian Code, which in the West was held in high esteem.  It was very influential among the Germanic nations, serving as the chief basis of their early legislation; it also paved the way for the more complete codification that followed in the Justinian Code, which superseded it.

To Justinian belongs the immortal glory of reforming the jurisprudence of the Romans.  “In the space of ten centuries,” says Gibbon, “the infinite variety of laws and legal opinions had filled many thousand volumes, which no fortune could purchase, and no capacity could digest.  Books could not easily be found, and the judges, poor in the midst of riches, were reduced to the exercise of their illiterate discretion.”  The emperors had very early begun to issue ordinances, under the authority of the various offices gathered into their hands; and these, together with the answers to appeals from the lower courts made to the emperors directly, or to the sort of supreme court which they established, were called imperial constitutions and rescripts.  Justinian determined to unite in one body all the rules of law, whatever may have been their origin; and in the year 528 appointed ten jurisconsults, among whom was the celebrated Tribonian, to select and arrange the imperial constitutions and rescripts, leaving out what was obsolete or useless or contradictory, and to make such alterations as the circumstances required.  This was called the Code, divided into twelve books, and comprising the constitutions from Hadrian to Justinian.  It was published in fourteen months after it was undertaken.

Justinian thereupon authorized Tribonian, then quaestor, vir magnificus magisteria dignitate inter agentes decoratus,—­“for great titles were now given to the officers of the crown,”—­to prepare, with the assistance of sixteen associates, a collection of extracts from the writings of the most eminent jurists, so as to form a body of law for the government of the empire, with power to select and omit and alter; and this immense work was done in three years, and published under the title of Digest, or Pandects.  Says Lord Mackenzie: