width and have a curved approximately wave like course the grooves 23 24 have drain openings 25 26 from which drain pipes 27 28 extend and are conducted to the exterior through the front face 3 short approximately horizontal grooves 29 30 are applied on both sides of the recess 22 each one of these is provided with an inflow opening 31 and an outflow opening 32 for a cleaning fluid the lateral wall 3 has a circular groove 33 which runs along its edges at an approximately constant distance an elastically expandable hose 34 lies in the groove 33 it can be connected by means of a supply line 35 with a pressure line not shown the front wall 3 is embodied mirror reversed in respect to the front wall 4 and a sealing plate 36 with the corresponding connections is embodied mirror reversed in respect to the sealing plate 20 in accordance with fig5 in which the front wall 4 with the associated sealing plate 36 is shown in a simplified manner additional throttle members 37 are provided in the edge zone adjoining the front wall 4 at both the inlet slit 7 as well as the outlet slit 8 the same applies to the opposite side not visible in fig5 as can be seen in fig1 such a throttle member is located in a longitudinal groove 38 which has been milled into the inner face of the base body 2 at a short distance below the sealing block 17 and extends from the edge over approximately a quarter of the total length it is made of a.