A cannula (10) for receiving surgical instruments for performing a surgical procedure on a body includes a tubular structure (12) defining a passage (16) through which the surgical instruments are inserted into the body. The tubular structure (12) includes an expandable portion (40) for enabling an increase in the cross-sectional area of the passage (16). The expandable portion (40) of the tubular structure (12) has a slot (100) and a guide member (114) disposed in the slot. The guide member (114) is movable from a first end (102) of the slot (100) toward a second end (104) of the slot to enable the cross-sectional area of the passage (16) to increase. The expandable portion (40) has a stop (106) between the first and second ends (102 and 104) of the slot (100) engageable with the guide member (114) to retain the guide member in a position relative to the slot and resist movement of the guide member from the position relative to the slot.