A vertebral rod assembly with first and second members that may rotate about a first axis. A lower rib extends laterally from the center support and supports a seat bottom fixed thereto. A concave part is formed in a downward surface of the mirror housing that is rotated relative to the mirror base in a state in which the downward surface faces to an upward surface of the mirror base. a tab extends inward from a back end of one of the first and second plates to block a back end of the ODD.