  We present a set of 6-12 micron ISOPHOT-S spectra of the general interstellar
medium of the Milky Way. This part of the spectrum is dominated by a series of
strong, wide emission features commonly called the Unidentified Infrared Bands.
The sampled area covers the inner Milky Way from l = -60 degr to +60 degr with
a ten-degree step in galactic longitude and nominal latitudes 0,+-1 degr. For
each grid position the actual observed direction was selected from IRAS 100
micron maps to minimize contamination by point sources and molecular clouds.
All spectra were found to display the same spectral features. Band ratios are
independent of band strength and Galactic coordinates. A comparison of total
observed flux in band features and IRAS 100 micron emission, a tracer for large
interstellar dust grains, shows high correlation at large as well as small
1-arcmin scales. This implies a strong connection between large dust grains and
the elusive band carriers; the evolutionary history and heating energy source
of these populations must be strongly linked. The average mid-infrared spectrum
of the Milky Way is found to be similar to the average spectrum of spiral
galaxy NGC 891 and the spectra of other spirals. The common spectrum can
therefore be used as a template for the 6-12 micron emission of late-type
spiral galaxies. Finally, we show that interstellar extinction only weakly
influences the observed features even at lambda = 10 microns, where the
silicate absorption feature is strongest.
