Any member of the fire department of any such city having a pension fund who shall, while in the performance of his duties in such fire department, become and is found, upon examination by a physician to be appointed by the board of trustees, to be physically or mentally disabled by reason of such service in such fire department to such extent that he is permanently unable to perform his accustomed tasks, shall be retired from service in such fire department by the board of trustees of the firemen’s pension fund upon the certificate of such physician and by a majority vote of the board.