Abstract: One aspect is a method for inserting an instrument into the gastrointestinal tract of a mammal, by emplacing at least a portion of a flexible liner to form within the intestine an inner tube within an outer tube, the inner tube defining a lumen lying within the intestine and having a distal end accessible via the anal opening from outside the body of the mammal, and thereafter passing the instrument into the lumen via its distal end. Another aspect is the combination of an elongate evertible liner, a liner support chamber having a proximal port and a distal port, the liner having its two ends attached respectively at the ports, and a pressurizer for increasing the pressure within the chamber to cause the liner to be everted, the liner comprising a material characterized by having a wall thickness to diameter ratio and a zero strain elasticity modulus whose product is less than 4.0 lb/inch.sup.2, preferably less than 1.0 lb/inch.sup.