ETHMOID BONE: CHARACTERISTICS, PARTS, FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2024

The ethmoid bone is a single, medium, symmetrical bone that is located in the skull forming its floor and contributes to the formation of the nasal and orbital cavities. It is located in front of the sphenoid bone, becoming the most anterior of the medial axis of the base of the skull, and behind the ethmoid notch of the frontal bone.

In order to be fully appreciated, it must be visualized in a totally disarticulated skull, due to its complexity and bone specialization.

Ethmoid bone from below

It has four ossification points, two lateral for the ethmoidal labyrinths and two medial for the central pieces (crista galli, lamina cribrosa and lamina perpendicular).

Its ossification is completed approximately between the 5th and 6th years and its definitive articulation with the vomer does not occur until approximately 45 years of age, from which it remains separated by a cartilaginous membrane until then.

Ethmoid characteristics

It is one of the most fractured bones with numerous cavities, notches and irregularities in the human body.

It articulates simultaneously with 13 bones of the face and the neurocranium and is the only bone in the skull that is not part of the cranial vault.

Ethmoid bone from the right side

It is the only bone at the base of the skull made up solely of cartilage that subsequently ossifies, making it extremely light and fragile in relation to its size.

In patients over 45 years of age, it is made up of entirely compact bone tissue except for the crista galli process where spongy tissue can be found.

Parts

Ethmoid bone from above

The ethmoid bone is made up of 4 parts: a perpendicular or vertical and middle lamina, a cribriform or horizontal lamina and two lateral bone structures called ethmoid labyrinths.

Perpendicular blade

This vertical lamina is divided by the cribriform lamina (horizontal) into two portions, an upper one, intracranial in the shape of a rooster's crest, called the “crista galli process” and a lower, extracranial one that participates in the conformation of the bony nasal septum. and that the perpendicular sheet itself is considered.

Crista galli process:

It has a triangular and vertical shape, it originates from the posterior border and rises forward, the falx cerebri is inserted into it.

The anterior border, when articulating with the frontal one, forms a canal called foramen cecum through which the dura mater extends.

Perpendicular plate proper:

It is square-shaped, thin, and extends downward. Its upper border corresponds to the base of the crista galli process.

The lower border is rough and bifurcated, where the cartilaginous portion of the nasal septum is housed. An anterior border that corresponds to the frontal nasal spine.

Its posterior border, more sinuous, bifurcated in its upper part to articulate with the crest of the sphenoid bone. In its lower part it corresponds to the vomer bone.

On the sides, there are grooves through which the olfactory nerves pass to finally enter the foramina of the cribriform plate.

Cribriform plate

It has a rectangular and elongated shape, arranged horizontally, it articulates with the front by means of the ethmoidal notch on the front.

The crista galli process divides this lamina into two portions. one right and one left, called olfactory grooves, which are perforated by multiple orifices through which the olfactory nerves, the anterior ethmoidal nerve and the anterior ethmoidal artery pass.

The olfactory bulbs rest on the upper face of the lamina, and the anterior face of the same forms part of the upper wall of the nostrils.

Ethmoidal labyrinths

On each side of the cribriform plate, two structures called ethmoidal labyrinths are projected downwards. They are complex structures in themselves, so up to 6 faces with different characteristics can be described.

On these faces, one can differentiate the anterior ethmoid cells on its anterior face where it articulates with the lacrimal bone, the middle ethmoid cells on its upper face where it articulates with the frontal bone, and the posterior ethmoid cells on its posterior face where it articulates with the body of the sphenoid.

Ethmoid cells can only be seen in a fully articulated skull, as they are formed by the union of structures in the articulation of two bones specifically designed for it.

In this ethmoidal labyrinth, the middle and superior nasal turbinates are located on the medial aspect. The inferior nasal turbinate is located on the palatine bone.

The superior nasal meatus is located between the middle and superior nasal turbinates and the middle meatus is located below the middle nasal turbinate.

On its underside, there is evidence of a thin, backward-facing structure called the uncinate process of the ethmoid, which contacts the ethmoid process of the lower nasal turbinate and subdivides the maxillary hiatus.

Behind the uncinate process, there is a raised and rounded structure, the ethmoidal bulla that is part of the ethmoid cells, between both structures the lunate hiatus is observed.

Features

The ethmoid bone, due to its morphological characteristics, has as its main function the “assembly” of all the bone structures of the skull.

It can be said that it shapes and couples the bone base for the correct functioning of the senses of vision, smell, hearing and taste.

Due to its position, it participates in the conformation of the nasal and orbital cavities, and due to its reliefs, notches, depressions and orifices, it allows the nasal air function for breathing, phonation and capture of odors.

It is specially designed to contain the nerve roots of the olfactory nerve and contribute to the formation and protection of air spaces.

Likewise, its galli crest process serves as an insert for the falx cerebri, also called “falx cerebri” that separates the brain, differentiating the right hemisphere from the left hemisphere.

References

1. Tim D. White. The human bone manual. Editorial Elsevier (2005) Pag. 112-113
2. Latarjet Ruiz Liard. Human anatomy. 4th Edition. Volume I. Editorial Médica Panamericana (2008) Pages 80-83
3. Henry Gray. Anatomy of the human body. (1918). Ethmoid bone. Recovered from: bartleby.com
4. S. National Library of Medicine. PubMed Health. Ethmoid Bone. Recovered from: ncbi.nlm.nih.gov
5. Virtual university. The skull. Bones of the brain case. Ethmoid bone. Recovered from: virtualuniversity.in