  The symmetry breaking of the actin network from radial to longitudinal
symmetry has been identified as the major mechanism for keratocytes (fish
cells) motility on solid substrate. For strong friction coefficient, the two
dimensional actin flow which includes the polymerisation at the edge and
depolymerisation in the bulk can be modelled as a Darcy flow, the cell shape
and dynamics being then modelled by standard complex analysis methods. We use
the theory of active gels to describe the orientational order of the filaments
which varies from the border to the bulk. We show analytically that the
reorganisation of the cortex is enough to explain the motility of the cell and
find the velocity as a function of the orientation order parameter in the bulk.
