An insertion instrument is provided including a tube having a proximal endadapted to be gripped by a user and a distal end adapted for insertion. A tipcontrolactuator is located at the proximal end, and the distal end of the tubeincludes aflexible portion. This flexible portion includes a spine and a plurality ofcircumferentially extending ribs extending from the spine. The ribs areaxiallyspaced apart from a distal tip toward the proximal end, with the ribs beingspacedapart by wedge shaped partial circumferential openings. The ribs have firstandsecond axial sides defined by the wedge-shaped openings, with the first axialside ofone of the ribs facing the second axial side of an adjacent one of the ribs.For each ofthe ribs, at least one of the first or second axial sides includes at leastone of anaxially extending projection or a recess and a facing one of the at least oneof thefirst or second axial sides of an adjacent one of the ribs includes at leastone of acomplementary mating recess or a corresponding axial projection. At least onetension wire extends from the tip control actuator to the distal tip
