A thin tube is extended telescopically in use to its full operating length. The open end section of the tube is used for mounting a medical instrument, and is conically tapered by two first welds (1) which run inclined at an angle (10) to the longitudinal axis of the flattened tube. At the end of the flattened tube the conically narrowed section has a second weld (2) which is vertically arranged relative to the longitudinal axis of the tube. In the middle section of the second weld is a stamped out section (4) which forms the opening (3) in the end of the tube.
