A medical instrument has a cannula, a seal disposed in said cannula, said seal has an opening for guiding a shaft of another instrument through said seal in a sealed manner. An expansion device serves for expanding said opening in said seal. Said expansion device has an expansion cone comprising a plurality of slats mounted pivotably on an annular body at a first end, a second end of said slots opposite to said first end being connected to said seal in an area of said opening. A flexible casing structure is provided accommodating said seal and said expansion device.