Abstract
The physical properties of the equatorial ``subcentral vicinity'' of quadrant IV of the Milky Way disk are investigated using a multi-line analysis of the observed ratios R[13/12]=13CO(J=1->0)/12CO(J=1->0) and R[21/10]=12CO(J=2->1)/12CO(J=1->0), and models with one and two components. Comparison with kinematical stability parameters reinforces our view of galactic structure and suggests a ``Smith Law'' like relationship for the massive star formation on galactic scales (i.e. kpc scale).
