  The spin structure of the system of quasifree fermions having total angular
momentum $J=1/2$ is studied in a consistently covariant approach. Within this
model the relations between the spin functions are obtained. Their particular
cases are the sum rules Wanzura - Wilczek, Efremov - Leader - Teryaev,
Burkhardt - Cottingham and also the expression for the Wanzura - Wilczek twist
2 term $g_{2}^{WW}$. With the use of the proton valence quark distributions as
an input, the corresponding spin functions are obtained. The resulting
structure functions $g_{1}$ and $g_{2}$ are well compatible with the
experimental data. Comparison with the basic formulas following from the
standard quark-parton model reveals the importance of the quark intrinsic
motion inside the target for the correct evaluation of the spin structure
functions.
