  An effective field theoretic description of the Isgur-Wise function in
heavy-meson transitions is presented which emulates soft interactions by a
fermion worldline with an infinitesimal self-intersecting loop. A
point-splitting regularization technique is used, which replaces pointlike
worldlines by ``ribbons'' in the sense of Witten. The calculated vertex
function is correctly normalized, does not depend on the heavy-quark mass, and
complies with different sets of recent experimental data of the ARGUS and CLEO
collaborations.
