ethmoidal air cell /ethmoi″dəl/ [Gk, ethmos, sieve, eidos, form] , one of the numerous small thin-walled cavities in the ethmoid bone of the skull. The cavities are lined with mucous membrane continuous with that of the nasal cavity and lie between the upper part of the nasal cavities and the orbits. Compare frontal sinus, maxillary sinus, sphenoidal sinus.
