Frontal bone

Frontal bone Definition

Frontal bone, also known as os Frontis, is a cranial bone making up the front part of the cranium that corresponds to the forehead region. It is derived from the Latin word “frons” means forehead.

The cranial vault bones are supported, made impervious and cushioned below with the structures designed to permit the drainage of cerebrospinal fluid. Hence, they are able to adjust the vital structures of the brain, nerves as well as the blood vessels. The frontal bone correlates to the front lobes of the brain, situated close to the forehead.

Frontal Bone Anatomy

The bone appears like a cockle-shell in form, and consists of three portions:

Frontal Bone Anatomy Picture

Vertical Portion or Squama frontalis

It corresponds to the forehead.

The exterior surface of the vertical portion (Squama frontalis) is convex and depicts the remnants of the frontal or metopic suture in the lower midline which make two halves of the frontal bone during infancy, a condition which may persist throughout life.

Nasal portion

It forms the root of the nose by articulating with the nasal bones and frontal process of the maxilla.

Orbital or Horizontal Portion (Pars orbitalis)

It forms the roofs of the orbital and nasal cavities.

This part contains two thin triangular plates known as orbital plates, which form the vaults of the orbits, and are separated from one another by a central gap which is called the ethmoidal notch.

Surfaces

The lower surface of every orbital plate is flat and rounded, and introduces, sideways, a slight void, the lacrimal fossa, for the lacrimal gland; adjacent the nasal portiont is a cavity named as fovea trochlearis. Sometimes a tiny trochlear spine is also present, for the connection of the cartilaginous pulley.

Borders

The vertical bone (squama) border is thick, indented and inclined at the sake of the inner table above where it lies upon the parietal bones, and at the sake of the outer table on either side, where this border experiences a counter lateral pressure from the bones, extending downwards into a triangular rough surface which is coherent with the great wing of the sphenoid. The posterior borders of the orbital plates are thin and indented, and coherent with the small wings of the sphenoid.

Structure

The zygomatic processes and squama are very thick being made up of diploic tissues existing in between the two closely packed laminæ; the frontal air sinuses exists in the region devoid of diploic tissues. The orbital portion is thin, translucent, consisting of compact bone providing the facility of instruments that can penetrate inside the cranium through this region.

Frontal Bone Anatomy Skull Image

Anterior View of Frontal bone

A round elevation known as Frontal eminence (tuber frontale) exists on both sides of metopic suture and more precisely 3 cm. above the supraorbital margin. These eminences are disproportional and uneven with varying sizes and are easily pronounced in young skulls. The bone area above them is smooth and covered by the galea aponeurotica. The region beneath the frontal eminences, two arched elevations, the superciliary arches are seen which are outstanding and easily visible medially and are connected with each other by a smooth protrusion called Glabella.

Superciliary Arch (or ridge)

It forms the eyebrow ridge and is present above the orbit as a smooth, arched elevation.

Supraorbital margin (or arch)

It exists as a curved narrow ridge that forms the superior edge of the orbit and separates the squama from the orbital part of the bone. Its lateral portion is sharp and pronounced and surmounts the eye. In that case, the medial part gets rounded to prevent the injury.

Supraorbital notch (or foramen)

It is present along the superior margin of the orbit as a small hole or a notch through which the supraorbital nerves and vessels emerges.

Glabella

A small elevation situated between the superciliary arches (eyebrows) and above the nasal bones (nose).

Orbital plate (or surface)

It is a part of the frontal bone that extends horizontally to form the roof of the orbit.

Zygomatic process

It exists as a narrow extension and the supraorbital margin ends laterally in it. It serves as a connecter between the frontal bone and the zygomatic bone.

Temporal line (a clearly distinguished line) runs upwards and backwards from the Zygomatic process , which divides into two parts the upper and lower temporal lines extending forward, in the articulated skull, with the analogous lines on the parietal bone. The anterior part of the temporal fossa lies beneath and in the exterior portion of these lines, which in turn gives rise to Temporalis muscle. On the midway of the supraorbital margins the squama stretch out downwards to a level below that of the zygomatic processes; this part is called the nasal part and introduces a rough interval known as nasal notch, which converse on both sides of the middle line with the nasal bone, and laterally integrate the frontal process of the maxilla and lacrimal. The middle portion of the frontonasal suture is called Nasion. The nasal process arises from the center of the nasal notch runs downward and forward beneath the nasal bones and frontal processes of the maxillæ. It strengthens the nose bridge thereby ending in a sharp spine. On either side of this is a small grooved surface which enters into the formation of the roof of the corresponding nasal cavity. The spine makes a portion of the septum of the nose, anteriorly fused with the crest of the nasal bones and posteriorly with the perpendicular plate of the ethmoid.

The interior surface of the squama is concave and the topmost portion of the middle line contains a vertical groove, the sagittal sulcus, whose edges joins inferiorly to form a ridge, the frontal crest; the superior sagittal sinus gets embedded in this sulcus, the falx cerebri originates from its margins and crest. The crest extends downwards as a small notch which in turn ends into a foramen, the foramen cecum, by articulation with the ethmoid. Its size alters considerably, and is impervious. The foramen when open serves as a passage for the vein extending from the nose to the superior sagittal sinus. At each end of the middle line the bone offers depressions for convolutions of brain, and various tiny furrows for the ventral branches of the middle meningeal vessels. Many small, eccentric fossæ may be seen on both side of the sagittal sulcus, for the admission of the arachnoid granulations.

Coronal view of Frontal bone

The Frontal Parnasal Sinuses exist as two large cavities situated above the orbits and lie deep to the superciliary arches. They are responsible for various acts like voice resonation, mucous secretion in the nasal cavity and keeping the skull light-weight.

Superior view of Frontal bone

It consists of Orbital Plate (or surface).

Lateral view of Frontal bone

It consists of Glabella, Zygomatic process and Orbital plate.

Function of Frontal Bone

The frontal bone is referred to as a beneficial bone. It is indeed a boon for the body as it plays a vital role in maintaining the proper shape, strength and structure of the forehead (skull) thereby protecting the brain from being easily damaged. Its thickness and flat shape permits free simple head movements and is responsible for prevention of brain injury while moving the head. This could not have been possible if its shape had been rounded.

The bone is relative to the nasal cavities which plays vital role of breathing. Any injury at this facial region can cause excessive brain damage as a result of Hypoxia (insufficient oxygen supply to the brain tissues). The frontal lobe shape also plays an important role in prevention of such injuries, as does the strength of the frontal bone.

Frontal bone Connections/Articulations

The frontal bone is connected or coherent with twelve bones:

  • The sphenoid
  • The ethmoid
  • Both parietals
  • The two nasals
  • Both maxillæ
  • Both Lacrimals
  • Both the Zygomatics

Frontal Bone Ossification

The ossification of frontal bone starts in membrane form from two primary centers, one for each half, noticeable above both the supraorbital margin only after the foetus becomes two month old. Ossification starts from these centers and continues upwards to form the analogous portion of the squama, and backward to give rise to the orbital plate. A pair of secondary centers serves as the ossification region for the spine on both the sides of the middle line. The nasal portion and the Zygomatic processes get ossified from the same centers.

At birth, the bone is divided into two parts by the frontal suture which is usually eradicated, slight remnants are seen at the lower region, by the eighth year, but occasionally persists throughout life. The frontal sinuses develop towards the beginning of the 2nd year.

References:

http://en.wikipedia.org/wiki/Frontal_bone

http://education.yahoo.com/reference/gray/subjects/subject/33

http://www.innerbody.com/image_skel01/skel40_new_skull.html

http://www.britannica.com/EBchecked/topic/220819/frontal-bone

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