  Using gauge invariant quark Green's functions, defined with path-ordered
gluon field phase factors along polygonal lines, and functional relations among
them, two compatible bound state equations of the Dirac type are established
for quark-antiquark systems, each relative to the quark or to the antiquark of
the system. The kernels of the bound state equations are defined through a
series of Wilson loop averages along closed polygonal contours and their
functional derivatives on them. A sufficient criterion for spontaneous chiral
symmetry breaking is derived, relating the Goldstone boson wave function in the
zero total momentum limit with the scalar part of the gauge invariant quark
two-point Green's function.
