For the purpose of providing and maintaining a fund to meet the payments of demands drawn for the payment of pensions and the expenses of the board, a trust fund account to be known as “county foresters, firewardens, and firemen’s retirement fund” shall be opened upon the books of the auditor and treasurer of any county adopting a system under this chapter. The contributions of the members and the sums made available by the board of supervisors for the purpose shall be credited to the fund.