At least one adjustment device (13) can engage with the floor and is mounted on the chassis (7) and can be activated and de-activated by a control (17). The fixture comprises wedge elements (43) concentric with the seat-column (5). The control comprises a hand-lever (19) connected by means of a pressure-axle (23) to the wedge element. A blocking element (45,45') inside the chassis runs at a preferably acute angle to the ground (9).
