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Ethmoid bone

German: Os ethmoidale

1 Definition

The ethmoid bone is a small, light skull bone not visible from the outside in the region of the skull base or the tectum of the nose.

2 Anatomy

The ethmoid bone forms part of the cranial cavity, the nasal cavity and the orbits. The bone can be divided into 4 sections: the cribriform plate, the perpendicular plate and the ethmoidal labyrinth.

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2.1 Cribriform plate

The cribriform plate or horizontal plate derives its name from its sieve-like appearance. In the middle of the plate, there is a thick, smooth, about triangular bony ridge, the so-called crista galli. The anterior edge of the crista galli articulates with the frontal bone. It has two small wings (alae) that communicate with the corresponding indentations of the frontal bone and form the foramen cecum. The posterior edge of the crista serves as insertion point for the falx cerebri.

The cribriform plate has a hollow point in both sides of the crista galli where the right and left olfactory bulb are laid up on the ethmoid bone. It is permeated by numerous fine bony canals (foramina) where the olfactory fibers draw from the nasal cavity into the cranial cavity to the olfactory bulb. The foramina in the middle of the hollow are very small, on the medial and lateral side, they are a little bit bigger.

In the front region of the cribriform plate, there is a small indentation on both sides, which the nasociliary nerve is passing through. From this indentation, a hollow is drawing from posterior direction towards the anterior ethmoidal foramen.

2.2 Perpendicular plate

The perpendicular plate is a thin, polygonal bone lamella which parts from the caudal side of the cribriform plate. It forms part of the nasal septum.

Its anterior edge articulates with the frontal bone and the crest of the nasal bone. The posterior edge articulates with the sphenoid bone in its cranial part, in the caudal part with the vomer. The inferior edge is thicker than the posterior one, and it serves as insertion point for the cartilage part of the nasal septum (cartilage of the septum). The surface of the perpendicular plate is predominantly smooth. Only in its upper part, it has dimples and canals, which are shaped by the olfactory fibers and draw to the cribriform plate.

2.3 Ethmoidal labyrinth

The paired labyrinth of the ethmoid bone consists of thin-walled air cells, the ethmoidal cells. They are arranged in three groups:

  • Anterior ethmoidal cells
  • Medial ethmoidal cells
  • Posterior ethmoidal cells

2.3.1 Facies superior

The superior surface of the labyrinth shows open ethmoidal cells an articulates with the frontal bone. This connection is interrupted by 2 small canals, the anterior ethmoidal canal and the posterior ethmoidal canal, which draw to the medial orbital wall and become visible there as the anterior ethmoidal foramen and the posterior ethmoidal foramen.

2.3.2 Facies posterior

The posterior surface also shows open ethmoidal cells, which, in the intact skull, are closed by the articulation with the sphenoid bone and the palatine bone.

2.3.3 Facies lateralis

The lateral surface is formed by a paper-thin, fragile cover plate, the lamina papyracea (orbital plate of the ethmoid bone). It covers the medial and posterior ethmoidal cells and forms a big part of the medial orbital wall. It articulates with the frontal bone on the upper side, on the lower side with the maxilla and the palatine bone, in front with the lacrimal bone and behind with the sphenoid bone. From the anterior part of the Facies lateralis, a curved bone lamella, the uncinate process, draws towards the caudal and dorsal direction. It forms a small part of the medial wall of the maxillary sinuses and articulates with the inferior nasal concha.

2.3.4 Facies medialis

The Facies medialis forms a small part of the lateral nasal wall. It consists of a thin bone lamella, that draws away fom the lower side of the cribriform plate and forms the medial nasal concha on the caudal side with its free, involuted edge. In its superior part, the Facies medialis has numerous dimples where the olfactory fibers pass through.

The medial nasal concha occupies the whole length of the Facies medialis and is convex towards above. Caudal and lateral to its concave downside, you can find the medial nasal meatus. Here is the termination point of the medial and - in the region of the infundibulum - the anterior ethmoidal cells.

The posterior part of the Facies medialis is divided by a narrow, diagonal fissure, the superior nasal meatus. This fissure is delimited by a thin bone lamella towards above and the middle, the superior nasal concha. This meatus is the termination point for the posterior ethmoidal cells.

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