the same separation each of the rows of teeth 9 is offset by a quarter pitch with respect to each of the other rows of teeth 9 in order to improve the dosing precision in a way known in its own right the rear casing section 3 forms blocking elements 4 which engage with the rows of teeth 9 the piston rod 8 is blocked by the engagement of the blocking elements 4 such that it is not possible to move the piston rod 8 counter to the advancing direction of the piston k movement of the piston rod 8 in the advancing direction however is permitted in order to advance the piston rod 8 the drive device 37 engages with the rows of teeth 9 of the piston rod 8 via slaving means 11 behind the blocking elements 4 the drive device 37 is formed by the sleeve shaped drive member 10 and the sleeved shaped operating element 18 the operating element 18 is pushed from behind onto the drive member 10 the connection between the drive member 10 and the operating element 18 is such that the operating element 18 cannot be axially moved relative to the drive member 10 but can be rotated about the longitudinal axis l relative to the drive member 10 the longitudinal axis l forms a translational axis of the drive device 37 and the driven device k 8 consisting of the piston k and the piston rod 8 and.