The ethmoidal air cells (also called ethmoidal sinuses) are small air-filled spaces positioned in the ethmoid. They are situated between the bony orbits on either side of the nasal septum.

At birth, a baby has 3 to 4 ethmoidal air cells that later develop into 5 to 15 paired cells. The total volume of these cells is 0.068 to 0.10 oz (2 to 3 ml).

The ethmoidal air cells are divided into three groups:

Anterior ethmoidal cells - the most anterior located cells; they are covered by the lacrimal bone; these cells are drained to the middle nasal meatus via the bony ethmoidal infundibulum;
Middle ethmoidal cells - the middle cells; they are covered by the orbital plate of the ethmoid; they are drained into the middle nasal meatus;
Posterior ethmoidal cells - the most posterior located spaces; they are also covered by the orbital plate of the ethmoid; these cells are drained into the superior nasal meatus.

The largest anterior ethmoidal air cell is called the bony ethmoidal bulla.

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Ethmoidal air cells