Jurors whose names appear in the list provided for under §§ 8.01-348 and 8.01-351 shall be used for the trial of cases, civil and criminal, to be tried during the term. The judge shall direct the selection of as many jurors as may be necessary to appear for the trial of any case. Any court shall have power to discharge persons summoned as jurors therein, or to dispense with their attendance on any day of its sitting. When by reason of challenge or otherwise a sufficient number of jurors summoned cannot be obtained for the trial of any case, the judge may select from the names on the jury list provided for by § 8.01-345 the names of as many persons as he deems necessary and cause them to be summoned to appear forthwith for the trial.