  A forward dispersion calculation is implemented for the spin polarizabilities
$\gamma_1, ... ,\gamma_4$ of the proton and the neutron. These polarizabilities
are related to the spin structure of the nucleon at low energies and are
structure-constants of the Compton scattering amplitude at ${\cal
O}(\omega^3)$. In the absence of a direct experimental measurement of these
quantities, a dispersion calculation serves the purpose of constraining the
model building, and of comparing with recent calculations in heavy baryon
chiral perturbation theory.
