Radius vs. velocity dispersion plot of MCs of the ¹³CO catalogue. The solid line represents the best fit for the relation σ_v = aR^β in the full radius range. The best-fit coefficients are a = 0.67 and β = 0.4. The dotted and dashed lines correspond to the relations derived in Paper I for the ¹²CO catalogue rescaled to this set of data: a = 0.7 and β = 0.08 for R < 2 (dotted line), and a = 0.5 and β = 0.59 for R > 2 (dashed line).