Distribution of HII regions was overlaid on the widely used image of the Milky Way spiral structure (NASA/JPL-Caltech/R. Hurt). The symbols are the same as those in Fig. 2. The outlines are the best-fitted four-arm model (left, also see the upper right panel of Fig. 10) and the best-fitted PL spiral arm model (right, also see the left panel of Fig. 11).