A pump comprising a tubular body delimiting internally a metering chamber and comprising an inlet orifice, an inlet valve, a stem mounted so as to be able to move inside the body and comprising an actuation tube delimiting an outlet duct, an insert assembled to the actuation tube and comprising a base placed inside the metering chamber and facing the inlet orifice, and a discharge orifice, an annular piston mounted so as to slide inside the body and on the stem, the piston and the base comprising annular surfaces suitable for forming a discharge valve, a ball-and-socket joint being arranged between the insert and the actuation tube.