  We discuss the spin structure of the proton in a light-cone treatment of the
chiral quark model. Based on the fact that the quark helicity ($\Delta q$)
measured in polarized deep inelastic scattering experiments is actually the
quark spin defined in the light-cone formalism, rather than the quark spin
($\Delta q_{QM}$) defined in the conventionally quark model (or in the rest
frame of the nucleon), we calculate the $x$-dependence of the polarized quark
distribution functions $\Delta q(x)$, and the polarized structure functions
$g_1 (x)$. Special attention is focused on the Melosh-Wigner rotation due to
the transversal motions of quarks inside the nucleon and its effects on the
bare quark input. It is shown that our results match the experimental data
well.
