  Quasi-nuclear systems, representing nuclei with variable size, are studied to
investigate the occurrence of the spin-orbit term in the nuclear mean field in
the transition from infinite nuclear matter to finite nuclei. Relativistic as
well as non-relativistic mean field calculations based on models for the
nucleon-nucleon ($NN$) interaction, which fit the $NN$ scattering data, are
considered. A very strong correlation between the strength of the spin-orbit
term and radius of the nuclear system is observed. The origin of the spin-orbit
term is analyzed by inspecting the contributions of the different partial waves
and various mesons in a One-Boson-Exchange model of the $NN$ interaction. The
influence of correlation effects and the enhancement of the small component of
Dirac spinors for nucleons in the nuclear medium is discussed.
