model. Other observables, namely the nucleon mean squared radii, the magnetic moments and the nucleon–$\Delta$ splitting are also computed. The calculation has been done taking into account the quark sea polarization effects. The final results, including rotational $1/N_c$ corrections, are compared with the existent experimental data and they are found to be in a good agreement for the constituent quark mass of $400$–$420\ MeV$.

**1. Introduction**

In the last years the most challenging problem in elementary particle physics seems to be solution of QCD in the low energy region. The main difficulties are due to the non-perturbative effects caused by growing effective coupling constant of the fundamental theory in the low energy limit. This prevents one from using the well-known main tool of theoretical physics — the perturbation theory. Because of this, the most intriguing features of QCD – confinement and chiral symmetry breaking – still remain conceptual and practical problems.

The above mentioned obstacles have initiated an increasing interest among the physicists in non-perturbative methods and effective low-energy models of hadrons. The effective models are expected to mimic the behaviour of QCD at energies below $\sim 1~GeV$ (confinement and/or chiral symmetry breaking) and to reproduce experimental data in this 