Abstract: A shaft for medical instruments, especially for guiding instruments into body cavities, has neighboring segments that are hollow on the inside and that form sections of the shaft. The segments can be adjusted in their positions by means of control wires so that various shaft curvatures can be set, whereby at least two groups of segments can be adjusted independently of one another. The segments of a first group, provided distally, are flexibly linked with each other. A tensioning device provides a variably adjustable spring force to the segments of the second group such that when under low tension, the segments are non-positively locked, and under high tension, the segments are positively locked.