Quark and gluon content of the proton spin

We examine the role of quarks and gluons in polarized deep-inelastic scattering in connection with the famous European Muon Collaboration experimental result. The gluon contribution generated by the axial anomaly is shown to have a well-defined physical meaning and its gauge-invariance properties are fully clarified. We also emphasize the role of the ghost contribution in the generalized Goldberger-Treiman relation and give a detailed calculation of the correction due to unequal quark masses and to ${\pi }^0-\eta -{\eta }^{\prime }$ mixing.