  We combine observations of spiral galaxies in the [CII] line at 158 micron,
made with the Long Wavelength Spectrometer aboard ISO, with previous data from
the Kuiper Airborne Observatory to study the origin of this line, which is the
main coolant of the interstellar medium at relatively low temperatures. We also
use HI and CO(1-0) observations of these galaxies and estimate the respective
line fluxes in the same beam as the [CII] observations. We confirm the
existence of a linear relation between the [CII] line intensity and the CO(1-0)
line intensity, that we extend to intrinsically fainter galaxies. The
dispersion around this relation is significant and due to variations in the
far-UV flux, thus in the star formation rate. We find that for the least active
galaxies of our sample, in terms of star formation, the rate of [CII] line
emission per interstellar hydrogen atom is similar to that in the Solar
neighbourhood. For those galaxies, most of the [CII] line emission comes
probably from the diffuse cold atomic medium. In more active galaxies,
considered globally, the average [CII] line emission is dominated by dense
photodissociation regions and to some extent by the warm ionized diffuse
medium. This is true in the central region of many spiral galaxies, and
probably even in the interarm regions of the most actively star-forming ones.
