The Dirac structure of the nucleon self-energy in symmetric nuclear matter
as well as neutron matter is derived from a realistic meson exchange model
for the nucleon-nucleon (NN) interaction. It is demonstrated that the effec
ts of correlations on the effective NN interaction in the nuclear medium ca
n be parameterized by means of an effective meson exchange. This analysis l
eads to a very intuitive interpretation of correlation effects and also pro
vides an efficient parametrization of an effective interaction to be used i
n relativistic structure calculations for finite nuclei.