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Other Terms: Ethmoidal bone, Os ethmoidale, Os ethmoïde
It is an unpaired bone wedged between the right and left orbits of the skull. The bone consists of a median vertical plate, a horizontal plate perforated by many small foramina, and bilateral pneumatic labyrinthine regions. The labyrinthine regions form most of the medial walls of the orbit and the superior and middle nasal conchae. This bone consists of thin laminae of compact bone surrounding many small air sinuses. These sinuses have mucous membranes that are continuous through openings and channels with the nasal mucosa. The thickest portion of the bone, the crista galli, is the superior projection of the perpendicular plate above the horizontal cribriform plate.
The term ethmoid comes from the Greek term ethmos meaning sieve. Galen called the bone the sieve-like bone because of the many small foramina that transmit the olfactory nerves to the nasal cavity.
The ethmoid bone articulates with thirteen bones, more than any other bone in the body. It articulates with the unpaired sphenoid, frontal, and vomer and the paired nasals, maxillae, lacrimals, palatines, and inferior nasal conchae. The articulations with the sphenoid are at the posterior margins of its cribriform, perpendicular, and orbital plates. It articulates with the frontal bone at the anterior margin of the crista galli and at the junction of the lateral cribriform margin with the superior margin of the orbital plate. The vomer articulates with the postero-inferior margin of the perpendicular plate. The paired nasal bones join at the midline and articulate with the anterior margin of the perpendicular plate. It articulates with the maxillae and orbital processes of the palatines at the inferior border of the orbital plate. The anterior borders of the orbital plates articulate with the posterior border of the lacrimal bones. Small, delicate uncinate processes from the nasal plates of the labyrinths descend inferiorly to join the ethmoidal process of the inferior nasal conchae.
The ethmoid ossifies endochondrally from three centers in the cartilage of the nasal capsule. Two centers appear bilaterally in the orbital lamina during the fourth fetal month. Each of these centers begins to ossify the orbital plate and then spreads medially to form the superior and middle nasal conchae. At birth the bone consists of weakly ossified paired labyrinths separated medially by cartilage. The center for the perpendicular plate and crista galli appears during the first year. These centers fuse in the second year at the base of the crista galli. The air sinuses begin to form before birth as small canals. The growing bone resorbs and the mucosal linings extend into the spaces. By the third to fourth year the air cells are well established.