For the administration of the criminal law, or for the defense of civil actions brought pursuant to chapter two hundred and fifty-eight, Suffolk county shall constitute the Suffolk district; Middlesex county, the northern district; Essex county, the eastern district; Norfolk county, the Norfolk district; Plymouth county, the Plymouth district; Bristol county, the Bristol district; Barnstable, Nantucket and Dukes counties, the Cape and Islands district; Worcester county, excluding the town of Athol, the middle district; Berkshire county, the Berkshire district; Hampden county, the Hampden district; and Franklin county, including the town of Athol, and Hampshire county, the northwestern district.
Massachusetts Legal Code