A pump including a first portion (2) with a metering cylinder (3) containing a plunger (4), and a second portion (7) engaging the surface of the first portion (2) in which the outlet of the cylinder (3) is located, and comprising a recess (10) located opposite the cylinder for delivering a liquid. A flexible impervious membrane (8) clamped between the two portions (2, 7) comprises an opening (13) facing the cylinder (3) and a resilient circumferential ring (12) engaging the bottom of the recess (10). A disc (14) in the central area of the recess engages the membrane (8), and a duct (6) linked to the cylinder (5) and having an outlet in the same surface of the first portion (2) as the cylinder (5) is connected to a liquid discharge tubing.