(a) When the office of sheriff is vacant and until his successor is qualified, when the sheriff is imprisoned or in cases in which the sheriff is a party, is interested or otherwise is incompetent to act, the summons must be directed to and executed by the coroner if such interest or cause appears on the face of the proceedings; and if such interest or cause does not appear on the face of the proceedings, the coroner must execute the summons, though directed to the sheriff, if the judge of probate, on proper showing by affidavit, directs the execution thereof by the coroner.
(b) When, in such cases, the coroner has not qualified or the office is vacant or when the coroner is absent from the county having no deputy therein, when the coroner is imprisoned or when he is a party to or interested in the action, the summons must be directed to and executed by a special coroner, to be appointed by the judge of probate.