  Given a Riemann surface with boundary S, the lengths of a maximal system of
disjoint simple geodesic arcs on S that start and end at the boundary of S
perpendicularly are coordinates on the Teichmueller space T(S). We compute the
Weil-Petersson Poisson structure on T(S) in this system of coordinates and we
prove that it limits pointwise to the piecewise-linear Poisson structure
defined by Kontsevich on the arc complex of S. As a byproduct of the proof, we
obtain a formula for the first-order variation of the distance between two
closed geodesics under Fenchel-Nielsen deformation.
