  A simple model for the hadronic contribution to the photon vacuum
polarization function $\Pi_{had}(q^2)$, for spacelike momenta, is presented.
For small momenta, the two loop contribution from the pseudoscalar meson octet
is computed from the chiral Lagrangian. The light quark contribution (which at
low momentum gives the ${\cal O}(q^6)$ counterterm in the chiral Lagrangian) is
calculated within a relativistic constituent quark model incorporating the
momentum dependence of the quark mass. The perturbative gluons of QCD are
included in a standard fashion. The total result is close to an estimate of
$\Pi_{had}(q^2)$ that is obtained directly from $e^+e^-\to hadrons$ data. We
further use our results for $\Pi_{had}(q^2)$ to calculate the ${\cal O}(e^4)$
hadronic contribution to lepton magnetic moments and to calculate
$\alpha_{QED}(M_Z^2)$. A simpler model of constituent quarks with momentum
independent masses gives less favourable results.
