The Ethmoidal Air Cells (cellulæ ethmoidales) consist of numerous thin-walled cavities situated in the ethmoidal labyrinth and completed by the frontal, maxilla, lacrimal, sphenoidal, and palatine.
They lie between the upper parts of the nasal cavities and the orbits, and are separated from these cavities by thin bony laminæ. On either side they are arranged in three groups, anterior, middle, and posterior. The anterior and middle groups open into the middle meatus of the nose, the former by way of the infundibulum, the latter on or above the bulla ethmoidalis. The posterior cells open into the superior meatus under cover of the superior nasal concha; sometimes one or more opens into the sphenoidal sinus. The ethmoidal cells begin to develop during fetal life.