Ethmoid Bone

Like the sphenoid bone the ethmoid bone has a very complex shape. Unlike the sphenoid, though, it contributes little to the cranial cavity but mostly to the facial structure. The superior portion is found midline in the anterior cranial fossa. Projecting inferiorly into face are three extensions that form the superior portion of the nasal cavity and part of the medial wall of the orbit. The two lateral extensions contain the ethmoid air cells which like all paranasal sinuses drain into the nasal cavity.

Parts – The superior portion where the three extensions connect, which is the part of the ethmoid bone forming the cranium, is known as the cribriform plate. This plate has numerous holes in it through which fibers of the first cranial nerve, the olfactory nerve pass. These are known as the olfactory foramina. Running between the foramina is a ridge of bone known as the crista galli. This acts as an anchoring point for the dura mater that passes between the left and right cerebral hemispheres.

Projecting inferiorly from the cribriform plate in the midline is the perpendicular plate of the ethmoid bone. This acts as part of the dividing wall between the two nasal passages.

Figure 16.

Figure 16.

Figure 17.

Figure 17.

The two projections laterally and inferiorly are the lateral masses of the ethmoid. The lateral wall of this section of the bone forms part of the medial wall of the orbit. The medial wall is separated from the lateral wall mostly by air filled cavities surrounded by thin plates of bone. The lateral wall is more complex than the medial wall as contains projections known as the concha or turbinates. The superior nasal concha and middle concha are part of the ethmoid bone. They project into the nasal cavity to help ensure that the air brought in through the nose is warmed by the surface blood vessels and filtered by the mucus made in the respiratory epithelium. By projecting into the cavity, they lessen the amount of open space and act to cause air turbulence which helps bring all the air in contact with the surface. Each of these conchae extend from the wall and project inferiorly leaving a space between it and the lateral wall of the nasal cavity. Each of these spaces is known as a meatus and named for the concha that forms it, so the superior concha forms the superior meatus.

Openings – The openings for cranial nerve I, the olfactory foramina were discussed earlier. The other openings are for drainage and empty in those area found in the individual meatus (This is meant as a plural. Believe it or not the plural of meatus is meatus) and an area superior and posterior to the superior meatus known as the sphenoethmoidal recess. This is continuous with the superior meatus and the sphenoidal sinus and the posterior ethmoid air cells drain into these areas. The middle meatus drains the remaining paranasal sinuses: the anterior ethmoid, the frontal and the maxillary. The inferior meatus is formed by a different bone and will be discussed with the inferior concha bone which forms it.