The lymphangion are lined by smooth muscle cells which when they contract contribute actively to lymph transport by propelling the lymph bolus downstream. Contractions are not rhythmic all along the collectors and often short sections composed of 2 to 5 contiguous lymphangions contract vigorously in an ordered sequence to propel the lymph in a centripetal direction. The contraction only starts when a lymphangion contains enough lymph to stretch its endothelium. Relationships between lymphatic transmural pressures and nitrous oxide contribute to the adjustment of lymphatic pumping. Recently, Cajal-like cells were identified inside the wall of the main collector, the thoracic duct. They are probably involved in the neuromuscular organization of lymph propulsion.