  The morphology of the Milky Way is still a matter of debate. In order to shed
light on uncertainties surrounding the structure of the Galaxy, in this paper,
we study the imprint of spiral arms on the distribution and properties of its
molecular gas. To do so, we take full advantage of the SEDIGISM survey that
observed a large area of the inner Galaxy in the $^{13}$CO(2-1) line at an
angular resolution of 28". We analyse the influences of the spiral arms by
considering the features of the molecular gas emission as a whole across the
longitude-velocity map built from the full survey. Additionally, we examine the
properties of the molecular clouds in the spiral arms compared to the
properties of their counterparts in the inter-arm regions. Through flux and
luminosity probability distribution functions, we find that the molecular gas
emission associated with the spiral arms does not differ significantly from the
emission between the arms. On average, spiral arms show masses per unit length
of $\sim10^5-10^6$ M$_{\odot} $kpc$^{-1}$. This is similar to values inferred
from data sets in which emission distributions were segmented into molecular
clouds. By examining the cloud distribution across the Galactic plane, we infer
that the molecular mass in the spiral arms is a factor of 1.5 higher than that
of the inter-arm medium, similar to what is found for other spiral galaxies in
the local Universe. We observe that only the distributions of cloud mass
surface densities and aspect ratio in the spiral arms show significant
differences compared to those of the inter-arm medium; other observed
differences appear instead to be driven by a distance bias. By comparing our
results with simulations and observations of nearby galaxies, we conclude that
the measured quantities would classify the Milky Way as a flocculent spiral
galaxy, rather than as a grand-design one.
