to a controlled device 37 for instance a pump for supplying and evacuating the pressurized medium illustratively the medium is water supplied or evacuated by a pump 37 the device is controlled by a control unit 38 the inset 36 runs over the full length of the rail 6 and thereby reliably cooperates with all heddles fig1 consequently the inset 36 is at least as long as the zone of the rail holding the heddles 10 in a variation of this embodiment the inset 36 receives compressed air as the pressurized medium in this instance the device 37 is a valve unit by which to connect the inset 36 to a source of compressed air when a pressurized medium is fed into the tubular inset 36 the latter constrains the heddles 10 to come into direct contact with the upper drive surface 15 of the rail 6 while simultaneously itself setting up contact between the lower drive surface 16 of the rail 6 and the heddles 10 thereby the heddles 10 are held in play less manner at the rail 6 and free displacement of the heddles 10 in their lengthwise direction a relative to the rail 6 is precluded impacts vibrations and noise are averted in this manner when the pressurized medium is evacuated from the tubular inset 36 the elastic inset 36 resumes its position shown in fig1 and thereupon the heddles 10 can be displaced along the rail 6 in problem free manner this approach may be required for instance when ruptured warps must be fixed during weaving the device 37 is so controlled by the control unit 38 that the tubular inset 36 is fed with a pressurized.