The almost t-shaped ethmoid bone divides the cranial and nasal cavities as the roof of the nose and centre of the anterior cranial fossa. Fibres of the olfactory nerve (CN I) exit from the cranial cavity through foramina of the cribriform plate into the nose.

• Superiorly, the crista galli is a midline projection into the cranial cavity and serves as the anterior attachment for the falx cerebri (a dural fold).
• Inferiorly, the perpendicular plate forms the top of the nasal septum.
• On both sides, each lateral mass (labyrinth) contributes its orbital plate to the medial orbital wall and inward projections known as the superior and middle nasal conchae to the lateral wall of the nasal cavity. Between the orbital plate and conchae, are the ethmoidal air cells (sinuses), one of four paranasal sinuses, from which mucus drains into the superior and middle meatus (beneath their corresponding conchae).