  The universal curve $\sigma /A$ of nuclear photoabsorption is investigated
within a Fermi gas model of nuclear matter. An energy range from pion threshold
up to 400 MeV is considered. The interactions between nucleon, pion, $\Delta
$-isobar and photon are considered in the non-relativistic approximation with
corrections of the order $1/M$ taken into account with respect to proton mass.
Analytical expressions are obtained, in which the influence of nuclear
correlations, two-nucleon contributions and relativistic corrections is studied
explicitely. An extension of the model calculation to nucleon knock-out
reactions is discussed.
