The ethmoidal sinuses or ethmoidal air cells of the ethmoid bone are two of the four paired paranasal sinuses. They are a variable in both size and number of small cavities in the lateral mass of each of the ethmoid bones and cannot be palpated during an extraoral examination. They are divided into the anterior, middle and posterior groups (see below). The ethmoidal air cells consist of numerous thin-walled cavities situated in the ethmoidal labyrinth and completed by the frontal, maxilla, lacrimal, sphenoidal, and palatine bones. They lie between the upper parts of the nasal cavities and the orbits, and are separated from these cavities by thin bony laminae.
Acute ethmoiditis in childhood and ethmoidal carcinoma may spread superiorly causing meningitis and cerebrospinal fluid leakage or it may spread laterally into the orbit causing proptosis and diplopia.