The ethmoid bone (also known as the ethmoid) is an unpaired bone of the skull that contributes to the medial wall of the orbit, and the nasal cavity, including the nasal septum.

The ethmoid bone has four parts:

Cribriform plate

The cribriform plate is situated in the horizontal plane, covering the ethmoidal notch of the frontal bone. The cribriform plate forms the roof of the nasal cavity and a floor part of the anterior cranial fossa of the internal cranial base.

This plate has a sieve-like structure because it has small openings called cribriform foramina (also known as olfactory foramina), which transmit the fibers of the olfactory nerves (CN I) from the olfactory epithelium (nasal cavity) to the brain (cranial cavity).

Perpendicular plate

The perpendicular plate is a part of the ethmoid situated in the sagittal plane below the cribriform plate.

The lower part of the perpendicular plate forms the posterior nasal septum, while the upper part rises above the cribriform plate as the crista galli. The crista galli provides an attachment for the falx cerebri.

Ethmoidal labyrinth

The paired ethmoidal labyrinth is situated in the sagittal plane below the lateral margin of the cribriform plate.

The ethmoidal labyrinth separates the orbit from the nasal cavity and features several structures, including:

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Ethmoid bone