Ethmoid Bone

Definition of Ethmoidal Bone

The ethmoidal bone is a small solitary cranial bone that separates the nasal cavity from the brain. The bone derived its name from the Greek term ‘ethmos’, which means sieve, owing to its porous and spongy texture.

Location of the Ethmoidal Bone

Situated at the center of the skull, precisely at the roof of the nose, between the two orbits.

Key Characteristics

Functionality

It constructs the walls of the eye socket, nasal cavity, nasal septum, and the anterior cranial fossa floor.

Structure

This cube-shaped bone comprises three sections – the cribriform plate, the perpendicular plate, and two ethmoidal labyrinths.

1. Cribriform plate

This segment of the ethmoidal bone rests within the ethmoidal notch of the frontal bone, forming the roof of the nasal cavity. The structure is named after the Latin term ‘cribriform’, denoting ‘perforated’, as this plate has numerous perforations, giving it a sieve-like appearance. It permits the passage of olfactory fibers from the nasal cavity to the anterior cranial fossa. The plate also contains a small vertical bony projection called crista galli, to which the falx cerebri (a crescent-shaped sheet of dura mater that separates the two cerebral hemispheres) attaches.

2. Perpendicular plate

It is a thin lamina descending from the cribriform plate, attaching inferiorly to the septal cartilage and forming the upper two-thirds of the nasal septum.

3. Ethmoidal labyrinths

The ethmoidal labyrinths are extensive masses located on both sides of the perpendicular plate. They consist of multiple thin-walled compartments known as ethmoidal cells, which constitute the ethmoidal sinus. The number of these cells increases with age. These labyrinths also form a part of the superior and middle nasal conchae.

Each labyrinth is composed of two bone sheets – an orbital plate and a medial sheet. The orbital plate is the lateral bone sheet that forms the medial wall of the orbit. Conversely, the medial sheet constitutes the upper lateral wall of the nasal cavity, from which the superior and middle nasal conchae extend into the nasal cavity.

The outer edges of these labyrinths are termed 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 inner wall of the eye cavity.

Muscle Attachment

Since the ethmoidal bone is a component of the orbit, it is associated with the following seven extraocular muscles:

1. Lateral rectus
2. Medial rectus
3. Superior rectus
4. Inferior rectus
5. Superior oblique
6. Inferior oblique

Connections

The ethmoidal bone connects with 5 paired and 3 unpaired bones:

1. Paired bones – Inferior nasal conchae, maxillae, lacrimal, palatine, and nasal bones.
2. Unpaired bones – Frontal, vomer, and sphenoid bones.

References

1. 1. Ethmoidal bone — Kenhub.com
   2. The Ethmoidal Bone — Teachmeanatomy.info
   3. Anatomy, Head and Neck, Ethmoidal Bone — Ncbi.nlm.nih.gov
   4. Ethmoidal bone — Radiopaedia.org
   5. Ethmoidal bone — Anatomystandard.com