  We present a new general dispersive formalism for evaluating the hadronic
light-by-light scattering contribution to the anomalous magnetic moment of the
muon. In the suggested approach, this correction is related to the imaginary
part of the muon's electromagnetic vertex function. The latter may be directly
related to measurable hadronic processes by means of unitarity and analyticity.
As a test we apply the introduced formalism to the case of meson pole exchanges
and find agreement with the direct two-loop calculation.
