  We use the Herschel Hi-GAL survey data to study the spatial distribution in
Galactic longitude and latitude of the interstellar medium and of dense,
star-forming clumps in the inner Galaxy. The peak position and width of the
latitude distribution of the dust column density as well as of number density
of compact sources from the band-merged Hi-GAL photometric catalogues are
analysed as a function of longitude. The width of the diffuse dust column
density traced by the Hi-GAL 500 micron emission varies across the inner
Galaxy, with a mean value of 1{\deg}.2-1{\deg}.3, similar to that of the 250um
Hi-GAL sources. 70um Hi-GAL sources define a much thinner disk, with a mean
FWHM of 0{\deg}.75, and an average latitude of b=0{\deg}.06, coincident with
the results from ATLASGAL. The GLAT distribution as a function of GLON shows
modulations, both for the diffuse emission and for the compact sources, with
~0{\deg}.2 displacements mostly toward negative latitudes at l~ +40{\deg},
+12{\deg}, -25{\deg} and -40{\deg}. No such modulations can be found in the
MIPSGAL 24 or WISE 22 um data when the entire source samples are considered.
The distortions revealed by Herschel are interpreted as large-scale bending
modes of the Plane. The lack of similar distortions in tracers of more evolved
YSOs or stars rules out gravitational instabilities or satellite-induced
perturbations, as they should act on both the diffuse and stellar disk
components. We propose that the observed bends are caused by incoming flows of
extra-planar gas interacting with the gaseous disk. Stars decouple from the
gaseous ISM and relax into the stellar disk potential. The time required for
the disappearance of the distortions from the diffuse ISM to the relatively
evolved YSO stages are compatible with star-formation timescales.
