move it into the initial position according to fig1 with the valve 32 the volumetric flow of liquid is therefore restricted as a function of the pressure prevailing in the balloon 16 and the volumetric flow of liquid is maintained substantially constant via the flow restrictor 53 in principle the liquid pump could be modified by providing only a device for maintaining substantially constant the volumetric flow of liquid dispensed from the elastic element or only a device for restricting the volumetric flow of liquid dispensed from the elastic element before using the mechanically operated liquid pump liquid is delivered through the luer lock valve 21 as a result of which the liquid passes into the balloon 16 and the filling level of the balloon can be read off through the transparent cap 29 on the basis of the markings 51 which are arranged in the transverse direction of the cap and which are a reference for the transverse expansion of the balloon as a function of its state of filling after the pump 1 has been filled and the pump has been attached to the patient via the hose 50 liquid is dispensed out of the pump through the valve 32 with elastic pretensioning of the expanded balloon 16 and this is done until the balloon has been completely emptied and bears on the core 11 the particularly simple design of the described liquid.