What is Ethmoid Bone
Ethmoid is a small unpaired cranial bone that separates the nasal cavity from the brain. The bone has got its name from the Greek ‘ethmos’, meaning sieve, due to its lightweight and spongy texture.
Where is the Ethmoid Bone Located
It is located in the center of the skull, specifically at the roof of the nose, and between the two orbits.
|How many are there in the human body||1|
|Articulates with||Maxillae, inferior nasal conchae, vomer, nasal, frontal, lacrimal, palatine, and sphenoid bones.|
It forms the walls of the eye socket, nasal cavity, nasal septum, and the floor of the anterior cranial fossa.
This cubical bone is made up of three parts – the cribriform plate, perpendicular plate, and two ethmoidal labyrinths.
1. Cribriform plate
Lying within the ethmoidal notch of the frontal bone, this part of the ethmoid bone forms the roof of the nasal cavity. The structure has derived its name from the Latin ‘cribriform’, meaning ‘perforated’, as this plate has several perforations, giving it a sieve-like appearance. The olfactory fibers from the nasal cavity pass through these openings, reaching the anterior cranial fossa. The plate also features a small, vertical bony projection called crista galli, where the falx cerebri (a crescent-shaped sheet of dura mater that separates the two cerebral hemispheres) is attached.
2. Perpendicular plate
It is a thin lamina that descends from the cribriform plate. Inferiorly it attaches to the septal cartilage of the nose and hereby forms the superior two-thirds of the nasal septum.
3. Ethmoidal labyrinths
The ethmoid labyrinths are large masses located on both sides of the perpendicular plate. They are made up of multiple thin-walled compartments known as ethmoidal cells which comprise the ethmoidal sinus. The number of these cells increases with age. These labyrinths also form a part of the superior and middle nasal concha.
Each labyrinth is composed of two sheets of bone – orbital plate and medial sheet. Orbital plate is the lateral sheet of bone, which forms the medial wall of the orbit. On the other hand, medial sheet makes up the upper lateral wall of the nasal cavity, from which the superior and middle nasal conchae descend into the nasal cavity.
The external edges of these labyrinths are referred to as the orbital lamina or lamina papyracea. This lamina connects with the frontal bone, palatine bone, lacrimal bone, and sphenoid bone, forming the major portion of the inside wall of eye cavity.
As ethmoid bone forms a part of the orbit, it is associated with the following seven extraocular muscles:
- Lateral rectus
- Medial rectus
- Superior rectus
- Inferior rectus
- Superior oblique
- Inferior oblique
The ethmoid bone articulates with 5 paired and 3 unpaired bones:
- Paired bones – Inferior nasal conchae, nasal, maxillae, lacrimal, and palatine bones.
- Unpaired bones – Frontal, vomer, and sphenoid bones.