Michigan Compiled Laws, Chapter 123. Local Government § 123.339

Sec. 9. If any such system shall be divided, each public corporation shall provide for the operation of its portion and for the proper proportion of any expenses common to the entire system or separate systems, including payment of interest and principal of any unrefunded bonds of the original issue and for the payment of those expenses properly chargeable to its separate system including the payment of any bonds assumed or separately issued by it.