As the hysteroscope is slowly advanced into the cervical canal under direct vision, the isthmus (the internal os), which is a flattened, narrowed circle, becomes clearer. The surface is smoother than that of the highly folded endocervix. In most multiparous women, the general rigid hysteroscope with an outer sleeve (7-mm OD) can be introduced through the isthmus without prior mechanical dilatation. In nulliparous or aged women, however, the cervical canal may have to be dilated to a diameter of 7 mm in advance, and then the outer sheath with an obturator is introduced beyond the level of the internal os. The obturator is removed and replaced with a hysteroscope. When the irrigating fluid returns clear, the flow is stopped by closing the drain valve and the uterine cavity is visualized in its totality. A detailed examination begins near the internal os and proceeds gradually upward to the fundus and the uterine cornua.