Massachusetts General Laws Part IV. Crimes, Punishments and Proceedings in Criminal Cases (Ch. 263-280) Ch. 277, § 4

If there is a deficiency of grand jurors, writs of venire facias may be issued to the constables of such towns as the court orders to return forthwith the further number of grand jurors required. In Middlesex county, letters of venire shall be issued to the jury commissioner as set forth in chapter two hundred and thirty-four A.