  We study the two-dimensional structure of thin transonic accretion discs in
the vicinity of black holes. We use the hydrodynamical version of the
Grad-Shafranov equation and focus on the region inside the marginally stable
orbit. We show that all components of the dynamical force in the disc become
significant near the sonic surface and especially important in the supersonic
region. Under certain conditions, the disc structure is shown to be far from
radial, and we review the affected disc properties, in particular the role of
the critical condition at the sonic surface: it determines neither the
accretion rate nor the angular momentum in the accretion disc. Finally, we
present a simple model explaining quasi-periodical oscillations that have been
observed in the infrared and X-ray radiation of the Galactic Centre.
