General structure
Anatomically, the ethmoid bone is divided into four interconnected parts. These include two unpaired structures and one paired structure (which comprises two symmetrical formations).
The bone is based on two plates intersecting at a right angle — horizontal and vertical. Massive paired complexes containing air cavities are suspended on the sides of them. Due to this structure, the bone is lightweight while forming extensive surfaces for the nasal cavity and orbits.
Unpaired plates
Cribriform plate (Lamina cribrosa) lies horizontally. It serves as a peculiar "roof" of the nasal cavity.
- It is perforated by numerous small openings (Foramina cribrosa) through which the fibers of the olfactory nerves enter the cranial cavity.
- The crista galli (Crista galli) rises on the upper surface of the plate, behind which lies the wing of crista galli (Ala crista galli).
Perpendicular plate (Lamina perpendicularis) is oriented vertically. It descends downwards from the cribriform plate along the midline and takes a direct part in forming the bony nasal septum, dividing the nasal cavity into two halves.
Paired part: ethmoidal labyrinths
Ethmoidal labyrinths (Labyrinthus ethmoidalis) are paired volumetric formations attached to the edges of the cribriform plate.
They consist of numerous thin-walled ethmoidal cells (Cellulae ethmoidales), which are classified into three groups: anterior, middle, and posterior. The outer surface of the labyrinths is represented by the smooth orbital plate (Lamina orbitalis), which forms the medial wall of the orbit.
On the inner (nasal) surface of the labyrinths, curved bony projections are located — the nasal conchae:
- Superior nasal concha (Concha nasalis superior).
- Middle nasal concha (Concha nasalis media).
- Note: sometimes the supreme nasal concha (Concha nasalis suprema) is anatomically distinguished.
Additional complex microstructures of the labyrinths include: ethmoidal bulla (Bulla ethmoidalis), uncinate process (Processus uncinatus), ethmoidal infundibulum (Infundibulum ethmoidale), and semilunar hiatus (Hiatus semilunaris).
Briefly: answers to topical questions
Which cranial bones does the ethmoid bone articulate with?
Sources directly confirm that the ethmoid bone is fitted into the ethmoidal notch of the frontal bone. Its perpendicular plate participates in the formation of the nasal septum, and the labyrinths participate in the formation of the walls of the nasal cavity and orbit. A direct list of bones with which the ethmoid bone articulates is not provided in the cited sources.
Where do the anterior, middle, and posterior ethmoidal cells open?
The anterior, middle, and posterior ethmoidal cells open into the corresponding nasal meatuses of the nasal cavity.
- Anterior and middle cells open into the middle nasal meatus (Meatus nasi medius).
- Posterior cells open into the superior nasal meatus (Meatus nasi superior).
These openings provide communication between the pneumatic cavities of the ethmoidal labyrinth and the nasal cavity.
Which bones participate in the formation of the bony nasal septum?
The bony nasal septum (Septum nasi osseum) is formed by the perpendicular plate of the ethmoid bone, vomer, nasal spine of the frontal bone, and the sphenoidal crest of the body of the sphenoid bone.
Into which nasal meatus does the semilunar hiatus (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 an important communication pathway for the paranasal sinuses: the frontal sinus (sinus frontalis) opens into its anterosuperior part, and the maxillary sinus (sinus maxillaris) opens into its posteroinferior part.
To memorize the spatial arrangement, imagine the letter "T" or a scale: the horizontal bar is the cribriform plate (with the crista galli on top), the vertical stem is the perpendicular plate (nasal septum), and the weights suspended on the sides are the ethmoidal labyrinths with cells.
Class and exam questions
What type of bone is the ethmoid bone?
This is an unpaired pneumatic bone of the skull.
How many parts does Os ethmoidale consist of?
Out of four parts: two unpaired plates (cribriform and perpendicular) and one paired part (two ethmoidal labyrinths).
What is the function of the foramina in the cribriform plate?
They serve for the passage of olfactory nerve fibers from the nasal cavity into the cranial cavity.
Where are the ethmoidal cells located?
They are located inside the paired ethmoidal labyrinths and are divided into anterior, middle, and posterior.
What else to study on this topic
- Topography of the foramina of the cribriform plate (Foramina cribrosa)
- Division of ethmoidal cells into anterior, middle, and posterior
- Structure of minor labyrinth formations: ethmoidal bulla and semilunar hiatus
- Uncinate process and ethmoidal infundibulum
- Variant anatomy: supreme nasal concha
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