The possible explanation of the spin-orbit coupling, in heavy nuclei, by second-order effects of the tensor forces is investigated. The various second order terms are described by 4 graphs, in the case of one particle outside closed shells. The main contribution comes from an exclusion effect: two particles of the closed shells cannot, by mutual excitation, jump into the orbit which is already occupied by the outside particle. This effect could account, at least partially, for the observed spin-orbit splitting. The case of a hole is also investigated, and shows the same kind of agreement. A comparison with other works is discussed.