All trusts held by a county shall be administered by a board of 3 trustees. The members of the board of trustees for a county shall be the county treasurer, one county commissioner to be chosen by the commissioners from among their number, and one member of the county convention to be appointed by the chairman of the county convention. The trustees shall elect one of their number bookkeeper. The bookkeeper shall keep the records and books of the trustees, and shall require a voucher before making any disbursement of funds from the trusts held by his county.