Corpus: Ethmoid bone

The ethmoid bone is a small, light skull bone located at the base of the skull or the nasal roof, which is not visible from the outside. It belongs to the viscerocranium.

The ethmoid bone contributes to the formation of the cranial cavity, the nasal cavity, and the orbit. It can be divided into four sections: the cribriform plate, the perpendicular plate, and the paired labyrinths.

2.1. Cribriform plate

The cribriform plate, also known as the horizontal plate, is named for its sieve-like appearance. A thick, smooth, roughly triangular bony projection called the crista galli protrudes from the center of the cribriform plate. The anterior edge of the crista galli articulates with the frontal bone. It has two small wings (alae) that correspond with depressions in the frontal bone, forming the foramen cecum. The posterior edge of the crista galli serves as an attachment point for the falx cerebri.

On either side of the crista galli, the cribriform plate has depressions where the right and left olfactory bulbs rest. The plate is perforated by numerous small foramina, through which the olfactory nerves pass from the nasal cavity to the olfactory bulb. The foramina in the central part of this fossa are very small, while those medially and laterally are somewhat larger.

In the frontal area of the cribriform plate, there is a small notch on each side, through which the nasociliary nerve runs. From this notch, a groove runs posteriorly to the anterior ethmoidal foramen.

2.2. Perpendicular plate

The perpendicular plate is a thin, polygonal bony lamella that extends from the caudal side of the cribriform plate and forms part of the nasal septum. Its anterior edge articulates with the frontal bone and the crest of the nasal bone. The posterior margin articulates with the sphenoid bone in the cranial section and with the vomer in the caudal section. The inferior border is thicker than the posterior border and serves as an attachment for the cartilaginous part of the nasal septum (nasal septum cartilage). The surface of the perpendicular plate is predominantly smooth, with fine pits and channels in the upper part where the olfactory nerves run into the cribriform plate.

2.3. Labyrinth

The paired labyrinth of the ethmoid bone consist of thin-walled pneumatic spaces, the ethmoid cells. These are arranged in three groups:

• Anterior ethmoid cells
• Middle ethmoid cells
• Posterior ethmoid cells

2.3.1. Superior surface

The superior surface of the labyrinth shows open ethmoid cells and articulates with the frontal bone. This junction is interrupted by two small canals, the anterior, and posterior ethmoidal canals, which run to the medial orbital wall and become visible there as the anterior and posterior ethmoidal foramina.

2.3.2. Posterior surface

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

2.3.3. Lateral surface

The lateral surface is formed by a thin, fragile plate called the lamina papyracea (orbital plate of the ethmoid bone). It covers the medial and posterior ethmoid cells and forms a large part of the medial orbital wall. It articulates with the frontal bone above, the maxilla and palatine bone below, the lacrimal bone in front, and the sphenoid bone behind. A curved bony lamella, the uncinate process, extends caudally and dorsally from the anterior part of the lateral surface, forming a small part of the medial wall of the maxillary sinus and articulating with the inferior nasal concha.

2.3.4. Medial surface

The medial surface forms part of the lateral nasal wall. It consists of a thin bony lamella that extends from the underside of the cribriform plate and forms the middle nasal concha caudally with its free, curled edge. In the superior part, the medial surface has numerous pits through which the olfactory nerves pass.

The middle nasal concha occupies the entire length of the medial surface and is convex towards the top. The middle nasal meatus is located caudal and lateral to its concave underside. The middle and, in the area of the infundibulum, the anterior ethmoid cells open in this area.

The posterior part of the medial surface is divided by a narrow, oblique fissure, the superior nasal meatus. This fissure is limited upwards and medially by a thin bony lamella, the superior nasal concha. The posterior ethmoid cells open into this duct.