frontal sinus, one of a pair of small cavities in the frontal bone of the skull that communicate with the nasal cavity and lie above the orbits. The frontal sinuses are situated behind the superciliary arches and are lined with a mucous membrane that is continuous with that of the nasal cavity. Each sinus opens into the anterior part of the middle meatus through the frontonasal duct. The frontal sinuses are absent at birth, become well developed between the seventh and eighth years, and reach their full size after puberty. Compare ethmoidal air cell, maxillary sinus, sphenoidal sinus.
