  Using the two-loop approximation of chiral perturbation theory, we calculate
the momentum and density dependent nuclear spin-orbit strength $U_{ls}(p,k_f)$.
This quantity is derived from the spin-dependent part of the interaction energy
$\Sigma_{spin} = {i\over 2} \vec \sigma \cdot (\vec q \times\vec p)
U_{ls}(p,k_f)$ of a nucleon scattering off weakly inhomogeneous isospin
symmetric nuclear matter. We find that iterated $1\pi$-exchange generates at
saturation density, $k_{f0}=272.7 $MeV, a spin-orbit strength at $p=0$ of
$U_{ls}(0,k_{f0})\simeq 35$ MeVfm$^2$ in perfect agreement with the empirical
value used in the shell model. This novel spin-orbit strength is neither of
relativistic nor of short range origin. The potential $V_{ls}$ underlying the
empirical spin-orbit strength $\widetilde U_{ls}= V_{ls} r_{ls}^2$ becomes a
rather weak one, $V_{ls}\simeq 17$ MeV, after the identification $r_{ls}=
m_\pi^{-1}$ as suggested by the present calculation. We observe however a
strong $p$-dependence of $U_{ls}(p,k_{f0})$ leading even to a sign change above
$p=200 $MeV. This and other features of the emerging spin-orbit Hamiltonian
which go beyond the usual shell model parametrization leave questions about the
ultimate relevance of the spin-orbit interaction generated by $2\pi$-exchange
for a finite nucleus. We also calculate the complex-valued isovector
single-particle potential $U_I(p,k_f)+ i W_I(p,k_f)$ in isospin asymmetric
nuclear matter proportional to $\tau_3 (N-Z)/(N+Z)$. For the real part we find
reasonable agreement with empirical values and the imaginary part vanishes at
the Fermi-surface $p=k_f$.
