Abstract: This invention is concerned with a head box in paper machines, including substantially an upper lip, a lower lip, a fibrous slurry flow straightening member connected to a given number of micrometrical control devices which micrometrically adjust the linearity of the flow straightening member, and a mechanism which can be driven by a manually actuated handwheel and which is connected to a hollow beam made integral with the micrometrical control devices whose control rods are supported by said hollow beam, whereby as the mechanism handwheel is actuated, the flow straightening member is subjected to displacements of a given amplitude along a wall adjacent to the upper lip, so that said flow straightening member is made to adjustably project relative to the wall of an amount ranging for instance from 5 to 15 mm, in order to modify the fibrous slurry turbulence.