A medical punch includes a longitudinally dividable anvil with a reduced cross-sectional dimension facilitating insertion of the anvil into the vessel wall to achieve a clean aperture in the vessel wall by cooperative shearing action of a cutting edge and an anvil edge. In another embodiment, the instrument incorporates a measuring device for determining the thickness of a vessel wall between an anvil and a distal end of a cylindrical structure which together form a caliper, and the measuring device can also constitute a medical punch by cooperative shearing action of a cutting edge and an anvil to achieve a clean aperture in the vessel wall.