The Ethmoid

If the sphenoid is the most difficult cranial bone to describe and invision, the Ethmoid is the second most difficult. It has a number of features and projections, but unlike the sphenoid it cannot be seen from various views of the skull. Like the sphenoid, it is a single bone.

The ethmoid bone is located in the anterior portion of the floor of the cranium between the orbits, where it forms the roof of the nasal cavity. An inferior projection of the ethmoid bone, called the perpendicular plate, forms the superior part of the nasal septum that separates the nasal cavity into two chambers. Each chamber of the nasal cavity is referred to as a nasal fossa. Flanking the perpendicular plate on each side is a large but delicate mass of bone riddled with ethmoidal air cells, collectively constituting the ethmoid sinus. A spine of the perpendicular plate, the crista galli, projects superiorly into the cranial cavity and serves as an attachment for the meninges covering the brain. On both lateral walls of the nasal cavity are two scroll-shaped plates of the ethmoid bone, the superior and middle nasal conchae, also known as turbinates. At right angles to the perpendicular plate, within the floor of the cranium, is the cribriform plate, which has numerous cribriform foramina for the passage of olfactory nerves from the epithelial lining of the nasal cavity.

To view a QuickTime VR movie of the ethmoid bone click
The ethmoid touches, or articulates with, the following bones:

• Sphenoid
• Frontal
• Maxillae
• Palatines
• Vomer
• Lacrimals