A medical instrument has a cannula and a seal disposed in the cannula. An expansion device serves for expanding an opening in the seal for guiding shafts of other instruments of greater diameter in a sealed manner through the expanded opening. The expansion device has an expansion cone with a plurality of slats mounted pivotably on an annular body at a first end thereof, a second end of the slats being connected to the seal in an area of the opening. The slats are connected at its first ends to the annular body via spherical heads engaged into corresponding openings in the annular body.