  The quantum mechanical operator for angular momentum is transformed from the
real plane into the complex plane. In doing so, the Cauchy-Riemann (C-R)
equations are interpreted as constraint conditions defining two distinct
domains where complex differentiation is permitted. It is shown each of these
domains contains an orbital angular momentum contribution plus an non-orbital
term that cancels out between them. It is further shown the field equations for
spinning quantum particles include C-R equations that restrict the particles to
a single complex constraint space. It is therefore proposed the non-orbital
term in the constraint space angular momentum is the source of the spin.
