If at any time before the ease is finally submitted to the jury, one of the regular jurors should die or become ill in such manner as to be unable to perform his duties, or if for just cause he is excused, the court shall direct his substitution by the alternate juror, if only one. If there should be more than one juror, the alternate shall be drawn by lot. When the ease is submitted to the jury the court shall excuse the alternate jurors who have not been used.
Puerto Rico Legal Code