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, which have reached us nearly in their original form, are the most valuable which 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. A treatise of Ulpian, preserved in the Vatican, entitled "_Tituli ex corpore Ulpiani_" also contains valuable information, as well as the "_Receptae Sententiae_" of Julius Paulus, his great contemporary, both of which works, as well as others of inferior importance, were lately published at Rome by Dr. Gneist, called "_Corpus Juris Romani Antejustinianii_." [Footnote: Mackenzie, p. 16.] 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 no great accession was made to Roman law, until Theodosius II. 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 This was called the Theodosian Code, which in the West was held in high esteem, although superseded shortly after in the East by the Justinian Code. [Sidenote: Justinian labors.] 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." [Footnote: Gibbon, ch. 44.] 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, 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. This was published in fourteen months after it was undertaken. [Sidenote: Tribonian.] [Sidenote: The code of Pandects.]