  The spin structure of the proton is investigated in the framework of an
extended quark potential model which in addition to the conventional
$3q$--structure also takes into account $(3q)(q\bar q)$--admixtures in the
proton wave function. For reasons of parity such admixtures contain an odd
orbital angular momentum thus allowing the proton spin to be shared among quark
spins and orbital angular momenta. We show that only certain admixtures are
suited for a significant reduction of the quark spin content of the proton as
suggested by the EMC--result. Within a Hamiltonian model quark spin
contributions to the proton spin down to $0.5$ can be reproduced easily.
