  We present HHT observations in the N=3-2 rotational transition of the CN
radical toward selected positions of the Trapezium region and of the molecular
Ridge in the Orion molecular cloud. Two of the positions in the Ridge were also
observed in the N=2-1 line of CN and 13CN. The N=3-2 CN lines have been
combined with observations of the N=2-1 and N=1-0 transitions of CN, and of the
N=2-1 of 13CN to estimate the physical conditions and CN abundances in the
molecular gas. We analyze in detail the excitation of the CN lines and find
that the hyperfine ratios of the N=3-2 line are always close to the Local
Thermodynamic Equilibrium (LTE) optically thin values even in the case of
optically thick emission. This is due to different excitation temperatures for
the different hyperfine lines. From the line intensity ratios between the
different CN transitions we derive H_2 densities of ~10^5cm^-3 for the
molecular Ridge and of ~3x10^6cm^-3 for the Trapezium region. The CN column
densities are one order of magnitude larger in the Ridge than in the Trapezium
region, but the CN to H_2 ratio is similar both in the Trapezium and in the
Ridge. The combination of the low CN column densities, high H_2 densities and
relatively high CN abundances toward the Trapezium region requires that the CN
emission arises from a thin layer with a depth along the line of sight of only
\~5x10^15cm. This high density thin layer of molecular gas seems to be related
with material that confines the rear side of the HII region Orion A. However
the molecular layer is not moving as expected from the expansion of the HII
region, but it is ``static'' with respect to the gas in the molecular cloud. We
discuss the implication of a high density ``static'' layer in the evolution of
an HII region.
