“And be it enacted, that all causes or issues appointed to be tried before any Circuit Court, shall and may be so tried before any one or more of the Judges of the Court of Justiciary when upon circuit; and at all trials before any Circuit Court the jury shall be taken from the lists prepared for the trial of criminal offences: Provided always that it shall be competent to either division of the Court of Session, if in their judgment it shall be considered necessary, to direct any causes or issues to be tried by any other Judge or Judges of the Court of Session, at any circuit town, and, if necessary for the trial of the same, to cause jurymen to be summoned in the manner provided by the before-recited acts.”