When a habeas corpus is issued to bring the body of any prisoner committed as aforesaid, unless the court issuing the same deems it wholly unnecessary and useless, the court shall issue a subpoena to the sheriff of the county where said person is confined, commanding him to summon the witnesses therein named to appear before the court at the time and place where such habeas corpus is returnable. It is the duty of the sheriff to serve such subpoena, if it is possible, in time to enable such witnesses to attend. It is the duty of the witnesses thus served with said subpoena to attend and give evidence before the court issuing the same on pain of being deemed guilty of a contempt of court and proceeded against accordingly by said court.