  Based on our sample of 10095 galaxies with bulge-disc decompositions we
derive the empirical B-band internal attenuation--inclination relation for
galaxy discs and their associated central bulges. Our results agree well with
the independently derived dust models of Tuffs et al., leading to a direct
constraint on the mean opacity of spiral discs of Tau_B^f = 3.8 +/- 0.7
(central face-on B-band opacity). Depending on inclination, the B-band
attenuation correction varies from 0.2 -- 1.1 mag for discs and from 0.8 -- 2.6
mag for bulges. We find that, overall, 37 per cent of all B-band photons
produced in discs in the nearby universe are absorbed by dust, a figure that
rises to 71 per cent for bulge photons. The severity of internal dust
extinction is such that one must incorporate internal dust corrections in all
optical studies of large galaxy samples. This is particularly pertinent for
optical HST comparative evolutionary studies as the dust properties will also
be evolving. We use the new results to revise our recent estimates of the
spheroid and disc luminosity functions. From our best fitting dust models we
derive a redshift zero cosmic dust density of rho_{dust} ~ (5.3 +/- 1.7) x
10^5, h M_{odot} Mpc^-3. This implies that (0.0083 +/- 0.0027), h per cent of
the baryons in the Universe are in the form of dust and (11.9 +/- 1.7), h per
cent (Salpeter-`lite' IMF) are in the form of stars (~58 per cent reside in
galaxy discs, ~10 per cent in red elliptical galaxies, ~29 per cent in
classical galaxy bulges and the remainder in low luminosity blue spheroid
systems/components). [Abridged]
