  We investigate the gravitational instability of galactic discs, treating
stars and cold interstellar gas as two distinct components, and taking into
account the phenomenology of turbulence in the interstellar medium (ISM), i.e.
the Larson-type scaling relations observed in the molecular and atomic gas.
Besides deriving general properties of such systems, we analyse a large sample
of galaxies from The HI Nearby Galaxy Survey (THINGS), and show in detail how
interstellar turbulence affects disc instability in star-forming spirals. We
find that turbulence has a significant effect on both the inner and the outer
regions of the disc. In particular, it drives the inner gas disc to a regime of
transition between two instability phases and makes the outer disc more prone
to star-dominated instabilities.
