If, before the conclusion of the trial, a juror becomes sick, so as to be unable to perform his duty, the court may order him to be discharged. In that case a new juror may be sworn and the trial begin anew, or the jury may be discharged and a new jury then or afterwards impaneled; provided, however, that where substitute jurors have been impaneled in the manner provided by law, such substitute juror or jurors shall be used as provided by the law authorizing the impaneling of substitute jurors.