The "accumulation fund" shall be the fund in which the contributions of the employers shall be accumulated, to which shall be credited all amounts transferred from the employees' fund upon a member's retirement, and against which shall be charged all benefits paid on account of retired members. If a retired member is restored to service as an employee prior to his normal retirement date, the actuarial equivalent of the benefit provided by his accumulated contributions shall be transferred to the employees' fund.
T.C.A. § 8-3440