The
ethmoidal sinuses or
ethmoidal air cells of the ethmoid bone are one 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.