Ethmoid bone

At a glance

English

Ethmoid bone

Latin (TA2)

Os ethmoideum

System

Bone

Region

Head

Side

Single, on the midline

About the ethmoid bone

The ethmoid sits deep in the front of the skull, behind the root of the nose and between the two orbits. Its name comes from the Greek for a sieve, after the perforated plate on its upper side. Roughly cube-shaped, it is very light, because most of it is made of thin bony sheets enclosing air cells. It takes part in four regions at once: the anterior cranial fossa floor, the roof and side walls of the nasal cavity, the nasal septum and the medial wall of each orbit.

The bone has three parts. The cribriform plate is a horizontal sheet that fills the ethmoidal notch of the frontal bone and roofs the nasal cavity; it is pierced by many fine holes for the filaments of the olfactory nerves. From its upper surface the crista galli rises in the midline and anchors the front of the falx cerebri. The perpendicular plate hangs down from the cribriform plate in the midline and forms the upper part of the nasal septum. On each side hangs a labyrinth, a block of thin-walled air cells grouped as anterior, middle and posterior. Its outer wall is the paper-thin orbital plate, part of the medial orbital wall, and its inner wall carries the superior and middle nasal conchae, with the uncinate process and the ethmoidal bulla shaping the drainage routes of the middle meatus.

The ethmoid meets the frontal and sphenoid bones of the braincase and, in the face, the nasal bones, maxillae, lacrimal bones, palatine bones, inferior nasal conchae and vomer. It ossifies within the cartilaginous nasal capsule: the labyrinths start before birth and are still small at birth, while the crista galli and perpendicular plate ossify in the first year and join them early in the second.

Because it is thin and porous the ethmoid breaks easily, typically from an upward blow to the nose in a fall or road crash. A fracture of the cribriform plate can let cerebrospinal fluid drip from the nose and give nasal bacteria a path to the meninges, and it may tear the olfactory filaments, causing loss of smell and blunting of flavour. A break in the orbital plate lets air from the nose pass into the orbit, which swells further when the person sneezes or blows the nose.

Function and supply

Function

Divides the nose from the brain above it while letting the olfactory nerves through, forms part of the nasal septum and the medial orbital walls, and its conchae widen the surface that warms and moistens inhaled air.

Nerve supply

Olfactory nerve filaments pass through the cribriform plate; the anterior and posterior ethmoidal nerves, from the nasociliary nerve, and nasal branches of the maxillary nerve supply the lining of its cells and nasal surfaces.

Blood supply

The anterior and posterior ethmoidal arteries from the ophthalmic artery, together with branches of the sphenopalatine artery from the maxillary artery.