A cannula includes a hollow cylindrical body which is open at one end and closed at the other end. A slot extends from the open end toward the closed end, terminating with an opening that is larger in width than the slot, the width being measured in a direction perpendicular to the longitudinal direction in which the slot extends. The opening allows the passage of a guide wire for relative sliding. The configuration of the cannula is such that the end of the wire coming out of the cannula through the opening cannot, once inserted into the openings, penetrate into the part of the cannula beyond the opening between the latter and the closed end of the cannula. Related methods for inserting a tube shaped probe into a tear duct are disclosed.