  We have carried out survey observations toward the Galactic plane at l~38 deg
in the 12CO and 13CO J=1-0 lines using the Nobeyama Radio Observatory 45-m
telescope. A wide area (0.8 x 0.8 deg) was mapped with high spatial resolution
(17"). The line of sight samples the gas in both the Sagittarius arm and the
inter-arm regions. The present observations reveal how the structure and
physical conditions vary across a spiral arm. We classify the molecular gas in
the line of sight into two distinct components based on its appearance: the
bright and compact B component and the fainter and diffuse (i.e., more
extended) D component. The B component is predominantly seen at the spiral arm
velocities, while the D component dominates at the inter-arm velocities and is
also found at the spiral arm velocities. We introduce the brightness
distribution function and the brightness distribution index (BDI, which
indicates the dominance of the B component) in order to quantify the map's
appearance. The radial velocities of BDI peaks coincide with those of high 12CO
J=3-2/12CO J=1-0 intensity ratio (i.e., warm gas) and H II regions, and tend to
be offset from the line brightness peaks at lower velocities (i.e., presumably
downstream side of the arm). Our observations reveal that the gas structure at
small scales changes across a spiral arm: bright and spatially confined
structures develop in a spiral arm, leading to star formation at downstream
side, while extended emission dominates in the inter-arm region.
