Ethmoid Bone

Q: Which bones form the bony nasal septum?

The bony nasal septum (Septum nasi osseum) is formed by the perpendicular plate of the ethmoid bone, the vomer, the nasal spine of the frontal bone, and the sphenoidal crest of the sphenoid bone.

Q: Into which nasal meatus does the hiatus semilunaris open?

The semilunar hiatus (Hiatus semilunaris) opens into the middle nasal meatus of the nasal cavity. This anatomical structure is located directly beneath the middle nasal concha. The semilunar hiatus serves as an important drainage pathway for paranasal sinuses: the frontal sinus (sinus frontalis) opens into its anterosuperior part, and the maxillary sinus (sinus maxillaris) opens into its posteroinferior part.

Os ethmoidale

For medical students2 min readUpdated 2026-10-10

The ethmoid bone (Os ethmoidale) is an unpaired, pneumatic bone of the neurocranium. It is a critical component of the facial skeleton, contributing to the formation of the nasal septum, the nasal cavity walls, and the medial walls of the orbits.

Bone TypeUnpaired, pneumatic

CompositionFour parts (two unpaired, one paired complex)

CavitiesContains pneumatic ethmoidal air cells

FunctionForms walls of the nasal cavity and orbit

General Architecture

Anatomically, the ethmoid bone is divided into four interconnected parts comprising two unpaired structures and one paired complex (consisting of two symmetrical structures).

The core of the bone consists of two intersecting plates oriented at a right angle: a horizontal plate and a vertical plate. Suspended from the lateral margins of the horizontal plate are massive paired complexes containing air-filled cavities. This structural design ensures lightness while providing expansive surface areas for the nasal cavity and orbits.

Unpaired Plates

The cribriform plate (Lamina cribrosa) is oriented horizontally and acts as the "roof" of the nasal cavity.

It is perforated by numerous small foramina (Foramina cribrosa) that transmit the olfactory nerve fibers into the cranial cavity.
Projecting superiorly from the midline of the plate is the crista galli, posterior to which lies the ala of the crista galli.

The perpendicular plate (Lamina perpendicularis) is oriented vertically. It descends inferiorly from the cribriform plate in the midline, directly contributing to the bony nasal septum and dividing the nasal cavity into right and left halves.

Paired Part: Ethmoidal Labyrinths

The ethmoidal labyrinths (Labyrinthus ethmoidalis) are paired, voluminous structures suspended from the lateral margins of the cribriform plate.

They consist of thin-walled ethmoidal cells (Cellulae ethmoidales), which are traditionally classified into three groups: anterior, middle, and posterior. The lateral surface of each labyrinth features a smooth lamina, the orbital plate (Lamina orbitalis), which forms the medial wall of the orbit.

On the medial (nasal) surface of the labyrinths project curved bony shelves known as nasal conchae:

Superior nasal concha (Concha nasalis superior).
Middle nasal concha (Concha nasalis media).
Note: Occasionally, a highest nasal concha (Concha nasalis suprema) is anatomically present.

Additional intricate microstructures of the labyrinth include the ethmoidal bulla (Bulla ethmoidalis), uncinate process (Processus uncinatus), ethmoidal infundibulum (Infundibulum ethmoidale), and semilunar hiatus (Hiatus semilunaris).

Mnemonic

To remember the spatial orientation, visualize the letter "T" or a scale: the horizontal crossbar is the cribriform plate (with the crista galli on top), the vertical stem is the perpendicular plate (nasal septum), and the suspended weights on either side are the ethmoidal labyrinths containing the air cells.

Frequently asked questions

Which skull bones articulate with the ethmoid bone?

The ethmoid bone fits into the ethmoidal notch of the frontal bone. Its perpendicular plate contributes to the nasal septum, and its labyrinths form parts of the nasal and orbital walls. A complete list of all articulating bones is broad, including the frontal, sphenoid, maxilla, lacrimal, palatine, nasal bones, vomer, and inferior nasal conchae.

Where do the anterior, middle, and posterior ethmoidal cells drain?

The ethmoidal air cells open into the respective meatuses of the nasal cavity.

Anterior and middle cells drain into the middle nasal meatus (Meatus nasi medius).
Posterior cells drain into the superior nasal meatus (Meatus nasi superior).

These openings provide ventilation and drainage between the pneumatic spaces of the ethmoidal labyrinth and the nasal cavity.

Which bones form the bony nasal septum?

The bony nasal septum (Septum nasi osseum) is formed by the perpendicular plate of the ethmoid bone, the vomer, the nasal spine of the frontal bone, and the sphenoidal crest of the sphenoid bone.

Into which nasal meatus does the hiatus semilunaris open?

The semilunar hiatus (Hiatus semilunaris) opens into the middle nasal meatus of the nasal cavity. This anatomical structure is located directly beneath the middle nasal concha. The semilunar hiatus serves as an important drainage pathway for paranasal sinuses: the frontal sinus (sinus frontalis) opens into its anterosuperior part, and the maxillary sinus (sinus maxillaris) opens into its posteroinferior part.

What type of bone is the ethmoid bone?

It is an unpaired pneumatic bone of the skull.

How many parts make up the os ethmoidale?

Four parts: two unpaired plates (cribriform and perpendicular) and one paired component (the two ethmoidal labyrinths).

What is the function of the foramina in the cribriform plate?

They allow the passage of olfactory nerve fibers from the nasal cavity into the cranial cavity.

Where are the ethmoidal air cells located?

They are located within the paired ethmoidal labyrinths and are subdivided into anterior, middle, and posterior groups.

Go deeper

Topography of the cribriform plate foramina (Foramina cribrosa)
Classification of ethmoidal air cells into anterior, middle, and posterior groups
Structure of labyrinth microstructures: ethmoidal bulla and semilunar hiatus
Uncinate process and ethmoidal infundibulum
Anatomical variants: the highest nasal concha