Mich. Comp. Laws § 290.552

Sec. 2. For the purposes of this act, the state is divided into 7 districts: (a) District 1 consists of the counties of Arenac, Bay, and Midland plus all counties north of Midland County that are not otherwise designated in a district. (b) District 2 consists of the counties of Gratiot and Saginaw. (c) District 3 consists of the county of Tuscola. (d) District 4 consists of the counties of Genesee, Lapeer, Macomb, St. Clair, and Sanilac. (e) District 5 consists of the eastern half of the county of Huron using highway 53 as the western boundary. (f) District 6 consists of the western half of the county of Huron using highway 53 as the eastern boundary. (g) District 7 consists of the counties of Montcalm, Kent, Isabella, Mecosta, Eaton, Ingham, Clinton, and Shiawassee plus all counties west and south of this district not otherwise designated in a district.