  Two different approaches to mass and structure of the nucleon are discussed
in recent works, ${\it viz.}$ (case 1) the QCD lagrangian evaluated via lattice
calculations and (case ii) spontaneous symmetry breaking mediated by the
$\sigma$ field. These approaches are complementary in the sense that the QCD
lagrangian makes use of the gluon content of the nucleon entering in terms of
the gluon trace-anomaly and ignores the effects of $q{\bar q}$ vacuum
polarization, whereas in spontaneous symmetry breaking masses are formed by
attaching $q{\bar q}$ pairs to the valence quarks, thus giving them a definite
mass which is named the constituent mass. By the same process the $q{\bar q}$
pairs of the vacuum polarization acquire mass and in this form are the elements
of the quark condensate, having an up-quark and a down-quark component. A
linear combination of these two components in the form
$\sigma=1/\sqrt{2}(u{\bar u} + d{\bar d})$ shows up as the $\sigma$ field. It
is shown that (case i) corresponds to an unstable nucleon configuration whereas
(case ii) corresponds to a stable nucleon configuration as observed in
low-energy photo-nuclear and pion-nuclear reactions.
