Frontal bone

Os frontale

The single bone of the forehead, which also roofs both eye sockets and forms the front of the cranial vault.

At a glance

English: Frontal bone
Latin (TA2): Os frontale
System: Bone
Region: Head
Side: Single, on the midline
Muscles on it: Temporalis

About the frontal bone

The frontal bone gives the forehead its shape and closes the front of the braincase. In the adult it is one bone, set in front of the two parietal bones and above the skeleton of the face. Its large upright plate, the squamous part, makes the forehead itself. Below and behind this, a horizontal orbital part made of two thin plates roofs the orbits and the front of the cranial floor. A small nasal part lies between the orbits. Its name is simply the Latin frons, forehead.

On the outer face, two rounded frontal eminences sit above the orbits, and below them the brow ridges curve over each orbital margin and meet at the smooth glabella. The margin of each orbit carries a notch or small foramen for the supraorbital nerve and vessels, and at its outer end it becomes the zygomatic process, from which the temporal line sweeps up and back. Inside, the squama is hollowed to fit the frontal lobes, with a midline groove for the superior sagittal sinus that narrows below into the frontal crest. The orbital plates are thin; beneath each lie the fossa for the lacrimal gland laterally and the trochlear fovea medially, and between the two plates the ethmoidal notch is filled by the ethmoid's cribriform plate. The frontal sinuses are hollowed out of the bone behind the brow ridges.

The thick, toothed upper border meets the parietal bones at the coronal suture, and lower down each side joins the greater wing of the sphenoid, while the back edges of the orbital plates meet the lesser wings. The nasal part meets the nasal bones, the frontal processes of the maxillae and the lacrimal bones, and the zygomatic processes meet the zygomatic bones. The bone forms in membrane from two halves, one centre above each orbit appearing late in the second month before birth. At birth the halves are still divided by the frontal (metopic) suture, which normally closes in early childhood but stays open in some adults.

Clinically, an open metopic suture can be mistaken for a fracture on a radiograph. A direct blow to the forehead may break the bone over the frontal sinus, and the thin orbital plates can be pierced by a sharp object driven up through the orbit, opening a route from the eye socket to the brain.

Function and supply

Function: Shields the frontal lobes and the eyes from above and gives the forehead and brow their shape; its temporal line anchors the temporal fascia and its lower orbital surface carries the pulley for superior oblique.
Nerve supply: Not supplied as a muscle is; the supraorbital and supratrochlear nerves, from the ophthalmic division of the trigeminal nerve, cross its outer surface to the forehead skin, and meningeal branches of the trigeminal nerve supply the dura on its inner face.
Blood supply: The supraorbital and supratrochlear arteries supply the outer surface, while the anterior branch of the middle meningeal artery and the meningeal twigs of the anterior ethmoidal artery supply the inner surface and diploe.

Parts of the frontal bone

Each number is marked on the model. Press one, here or on the model, to see it named in place.

Ethmoidal notchIncisura ethmoideaThe four-sided gap between the two orbital plates on the underside of the bone. In the intact skull the ethmoid's cribriform plate fills it, and its edges carry half-cells that close the tops of the ethmoidal air cells.
Foramen caecum of frontal boneForamen caecum ossis frontalisA small pit at the lower end of the frontal crest, turned into a hole by the ethmoid just in front of the crista galli. In children it may carry a small vein from the nose to the superior sagittal sinus, but it is often closed in adults.
Fossa for lacrimal glandFossa glandulae lacrimalisA shallow hollow on the underside of each orbital plate, just behind the outer part of the orbital rim. The orbital lobe of the lacrimal gland rests in it.
Frontal crestCrista frontalisA midline ridge low on the inner surface of the squama, where the two lips of the sagittal groove come together. The front end of the falx cerebri is fixed to it.
Frontal eminenceEminentia frontalisOne of two rounded bulges on the forehead, roughly 3 cm above each orbital rim and to each side of the midline. They mark where bone first formed in each half and tend to be more obvious in children.
Frontal part of orbital marginPars frontalis marginis orbitalisThe curved upper rim of each orbit, sharp along its outer two-thirds and more rounded towards the nose. It carries the supraorbital notch or foramen for the supraorbital nerve and vessels.
GlabellaThe smooth, slightly flattened area of bone between the brow ridges, just above the root of the nose. It lies under the skin between the eyebrows and is a standard point for measuring the skull.
Internal surface of squamous part of frontal boneFacies interna squamae frontalisThe concave inner face of the forehead plate, lying against the dura over the frontal lobes. It carries the sagittal groove and frontal crest in the midline and faint impressions of the brain's folds and of meningeal vessels.
Nasal margin of frontal boneMargo nasalis ossis frontalisThe rough, toothed lower edge of the nasal part, between the two orbits. The nasal bones meet it in the middle and the frontal processes of the maxillae on either side.
Nasal part of frontal bonePars nasalis ossis frontalisThe small middle portion between the orbits, below the glabella. It forms the root of the nose and carries the nasal margin and nasal spine.
Nasal spine of frontal boneSpina nasalis ossis frontalisA pointed midline projection from the underside of the nasal part. It sits behind the nasal bones, props them up and adds a small piece to the top of the nasal septum.
Opening of frontal sinusApertura sinus frontalisThe outlet in the floor of each frontal sinus, close to the midline. Mucus leaves through it and runs down a narrow channel to the middle meatus of the nose, under the middle concha.
Orbital part of frontal bonePars orbitalis ossis frontalisThe roughly horizontal portion of the bone, made of the two orbital plates and the ethmoidal notch between them. It forms most of the roof of each orbit and much of the floor of the anterior cranial fossa.
Orbital plate of frontal boneLamina orbitalis ossis frontalisEach thin, triangular sheet of bone that roofs an orbit. The frontal lobe rests on its upper side and the eye lies below it; it is thin enough to break when a sharp object enters the orbit.
Orbital surface of frontal boneFacies orbitalis ossis frontalisThe smooth, concave lower face of each orbital plate, looking down onto the eyeball and its muscles. The lacrimal fossa lies at its outer front corner and the trochlear fovea at its inner front corner.
Parietal margin of frontal boneMargo parietalis ossis frontalisThe thick, deeply toothed back edge of the squama. It locks with the two parietal bones at the coronal suture.
Sphenoidal margin of frontal boneMargo sphenoideus ossis frontalisThe rough, triangular area low on each side of the squama, behind the zygomatic process. It joins the greater wing of the sphenoid bone.
Squamous part of frontal boneSquama frontalisThe large, curved upright plate that forms the forehead. Its outer face bears the eminences, brow ridges and glabella, its inner face faces the frontal lobes, and it encloses the frontal sinuses low down near the midline.
Temporal lineLinea temporalisA curved ridge that runs up and back from the zygomatic process across the side of the squama. It continues onto the parietal bone and marks the top of the temporal fossa, where the temporal fascia attaches.
Temporal surface of frontal boneFacies temporalis ossis frontalisThe small area on the side of the bone below and behind the temporal line. It forms the front of the temporal fossa and gives origin to part of temporalis.
Trochlear foveaFovea trochlearisA small dip near the front inner corner of the orbital roof, close to the rim. A loop of fibrocartilage fixed here acts as a pulley for the tendon of superior oblique.
Zygomatic process of frontal boneProcessus zygomaticus ossis frontalisThe strong projection at the outer end of the orbital rim. It meets the frontal process of the zygomatic bone to complete the lateral edge of the orbit.

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At a glance

About the frontal bone

Function and supply

Related structures

See the frontal bone where it really sits, with every layer of the body around it.