  (Abridged) We present the analysis of the main FIR fine structure lines
emission in NGC1313 and NGC6946. We calculate that a component probably
associated with the diffuse disks contributes <~40% in N6946 and ~30 % in N1313
to the total [CII] emission. The main PDR physical parameters responsible for
the neutral atomic gas emission in N1313 and N6946 do not significantly differ
from what Malhotra etal (2001) found by modelling the integrated emission of a
sample of 60 normal galaxies,although there are evidences for a beam averaged
contribution of a less active component inside NGC6946 higher than its
contribution in the integrated emission of normal galaxies. CO and [CII] in
N6946 are well correlated with a mean [CII]/CO ratio similar to that of the
normal galaxies sample. In N1313 the [CII]/CO seems to systematically increase
from the North to the south, along the S-shaped spiral arm, indicating much
more inhomogeneous conditions than in N6946. HI and [CII] in N6946 are
completely de-correlated, probably because they arise from different gas
components. In N1313 we successfully detect two distinct gas components: a
cirrus-like component where HI and [CII] are weakly correlated as observed in
our Galaxy, and a component associated with dense PDRs completely de-correlated
from HI as observed in N6946.Finally, we find that the HI residing in dense
PDRs and presumably recently photo-dissociated, constitutes a few % of the
total HI. In turn, this dense gas component produces most of the [CII] emission
emitted by the atomic neutral medium.
