The ethmoid bone (also ethmoidal bone, ethmoid, latin: os ethmoidale) is an unpaired bone of the skull that contributes to the medial wall of the orbit, the nasal cavity and the nasal septum. The ethmoid bone includes the cribriform plate with openings that transmit the olfactory nerves (CN I), and it also houses paranasal sinuses called the ethmoidal air cells.
The ethmoid bone has four parts:
- cribriform plate,
- perpendicular plate,
- ethmoidal labyrinth (2).
Cribriform plate of ethmoid
The cribriform plate of the ethmoid is a part of the ethmoid bone situated in the horizontal plane, covering the ethmoidal incisure. It forms the roof of the nasal cavity and part of the anterior cranial fossa of the internal cranial base. The cribriform plate has small openings which transmit the fibers of the olfactory nerves (CN I) from the olfactory epithelium (nasal cavity) to the brain (cranial cavity).
Perpendicular plate of ethmoid
The perpendicular plate of the ethmoid is a part of the ethmoid bone 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 paired ethmoidal labyrinth is a part of the ethmoid bone situated in the sagittal plane below the lateral margin of the cribriform plate of the ethmoid. The ethmoidal labyrinth separates the orbit from the nasal cavity. The ethmoidal labyrinth of the ethmoid features several structures:
- ethmoidal air cells (anterior, middle, posterior),
- orbital plate,
- superior nasal concha,
- middle nasal concha.
The ethmoidal air cells are air filled spaces in the ethmoidal labyrinth of the ethmoid bone belonging to the paranasal sinuses. There can be distinguished anterior, middle, and posterior ethmoidal air cells.
The orbital plate of the ethmoidal labyrinth is a thin bony plate found in the ethmoid bone, covering the ethmoidal cells from the orbital side and forming the lateral surface of the labyrinth.
The nasal conchae are curved bony plates, which form the passages of the nasal cavity - the nasal meatuses. The conchas cover the ethmoidal cells from the side of the nasal cavity forming the medial surface of the labyrinth. There are three nasal conchas on each lateral wall of the nasal cavity:
- superior nasal concha,
- middle nasal concha,
- inferior nasal concha.
The superior nasal conchae and the middle nasal conchae are a part of the ethmoid bone, while the inferior nasal conchae are separate bones.