Hiatus semilunaris

The hiatus semilunaris is a halfmoon-shaped groove in the lateral wall of the nasal cavity. It is bounded inferiorly and anteriorly by the sharp concave margin of the uncinate process of the ethmoid bone, superiorly by the ethmoidal bulla, and posteriorly by the ethmoidal process of the inferior nasal concha. Following the curve anteriorly leads into the infundibulum of the frontonasal duct, which drains the frontal sinus.

The anterior ethmoidal cells open into the front part of the infundibulum; in slightly over 50% of subjects, this is directly continuous with the frontonasal duct from the frontal air sinus. When the anterior end of the uncinate process fuses with the front part of the bulla, however, this continuity is interrupted and the frontonasal duct then drains directly into the anterior end of the middle meatus.