Pis24 is the main excitation source of this shell, since most of its massive stars are seen in projection onto the northern part of the cavity.

The northwestern section of the cavity is sharply bounded by the bright  region G353.2+0.9 (this region is analyzed in Sect.5).

The western boundary of G353.12+0.86 (near 172416, –341138) is thick, with an intricate net of filaments detected in optical lines. The  filaments are located in the inner part of the wall (Fig. 1). The radio continuum image (Fig. 5) displays faint and diffuse emission coincident in position with the area where the  and  filaments are distributed. Emission at 8 $\mu$m reveals a complex network of filaments, mixed with those observed at   (Figs. 1 and 7). The emission distribution at different wavelengths in this region suggests that PDRs and optical filaments at different distances along the western wall of the shell are observed.

The southweastern rim of the cavity, near 172430,$-$3419, is sharp and bright in optical lines, in the radio continuum, and at 8 $\mu$m (Figs. 1, 5, and 7). A close inspection of Fig. 1 shows that the emission in the different optical bands does not coincide in position, being the bright  emission 