A cannula assembly (10) for performing a surgical procedure includes a cannula defining a longitudinal axis (a) and having a longitudinal passageway (29) extending therethrough, a seal for substantially sealing the longitudinal passageway within the cannula in the absence of an object being received through the passageway and an elongated membrane (30) disposed within the longitudinal passageway of the cannula, The elongated membrane is secured to the cannula at respective ends of the elongated membrane to define an annular space (38) between the elongated membrane and the cannula. The cannula has a throughhole (42) in communication with the annular space whereby, when the cannula is positioned within an insufflated body cavity, insufflation gases pass through the throughhole to expand the annular space to thereby cause the elongated membrane to form a seal about an object inserted therethrough.
