in the profiles of Sect. \[coldensprof\] below correspond to such spatial estimates and should thus be taken as upper limits. Note that in the densest areas, we can distinguish between small-scale structure and instrumental noise or mid-IR emission fluctuations by checking for the presence on small scales of cold, IR-absorbing condensations in high-resolution millimeter continuum maps (see 2.3 below).

Millimeter observations
-----------------------

In order to check that the dark patches seen in the ISOCAM images (see Fig. \[absorption\]) were actually due to the presence of cold absorbing dense cores/condensations, and not to small-scale fluctuations of the mid-IR background and/or foreground, we carried out 1.3 mm continuum observations of a selected subset of our fields (see Table \[mm\]). We used the IRAM 30 m telescope equipped with the 37-channel MPIfR bolometer array in March 1998 and March/April 1999 to map the 1.3 mm dust emission of those fields that presented clear and compact absorption-like features in the ISOCAM images. On-the-fly 1.3 mm maps were taken in the dual-beam raster mode with a scanning velocity of 4$\arcsec$/sec, a spatial sampling of 2$\arcsec$ in azimuth and 4$\arcsec$ in elevation, and a chopping frequency of 2 Hz. The chop throw was set to 60$\arcsec$ 