An instrument comprises a flexible body comprising a proximal portion, a distal portion, and an inner sheath within the proximal portion. The inner sheath comprises a wall and a plurality of conduits extending through the wall within the proximal portion. A distal end of each conduit terminates proximal to the distal portion. The inner sheath further comprises a plurality of protrusions within the proximal portion. Each protrusion surrounds a portion of a corresponding conduit. The flexible body further comprises a steerable tube extending within the distal portion. The steerable tube comprises a wall and a plurality of grooves in the wall. The instrument further comprises a plurality of tendons extending from the proximal portion, beyond the distal ends of the plurality of conduits, and into the distal portion. In the proximal portion, each tendon extends through a corresponding conduit. In the distal portion, each tendon extends within a corresponding groove.