The role of external compression and movement in lymph propulsion in the sheep hind limb.

1. Pressure fluctuations and lymph flow were measured in metatarsal lymphatics in anaesthetized sheep. 2. Intermittent compression significantly increased lymph flow when this was applied over the hoof but did not increase flow significantly when applied over the metatarsal region. 3. In a second preparation a 15 cm length of metatarsal lymphatic was cannulated at both ends and measurements were made of the ability of the duct to pump saline from an inflow reservoir through an outflow at the same height. 4. In the absence of external forces fluid was propelled by the lymphatic's intrinsic contractions but when intermittent compression was applied over the metatarsal region flow increased almost fourfold. 5. When animals with the doubly cannulated duct were allowed to recover, the effect of normal limb movements on fluid propulsion was examined. Under these conditions flow only occurred in response to intrinsic lymphatic contractions and appeared to be unaffected by the animal moving round the cage. 6. These results suggest that the effects of external forces on lymph flow are more dependent on compression of tissues in the lymphatic drainage area than on compression of the main lymphatic ducts. External compression can increase fluid propulsion by these vessels but, since forces of adequate magnitude appear not to be encountered in normal hind-limb movements, lymph propulsion in this region must depend on intrinsic lymphatic pumping.