Frontal bone

The frontal bone is a bone in the human skull. The bone consists of two portions.[1] These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, part of the bony orbital cavity holding the eye, and part of the bony part of the nose respectively. The name comes from the Latin word frons (meaning "forehead").

Frontal bone
Frontal bone lateral3
Position of the frontal bone (highlighted in green).
Skull sword trauma
19th Century skull showing sword-blade trauma on frontal bone.
Details
ArticulationsTwelve bones: the sphenoid, the ethmoid, the two parietals, the two nasals, the two maxillæ, the two lacrimals, and the two zygomatics
Identifiers
LatinOs frontale
MeSHD005624
TAA02.1.03.001
FMA52734
Anatomical terms of bone

Structure

The frontal bone is made up of two main parts. These are the squamous part, and the orbital part. The squamous part marks the vertical, flat, and also the biggest part, and the main region of the forehead. The orbital part is the horizontal and second biggest region of the frontal bone. It enters into the formation of the roofs of the orbital and nasal cavities. Sometimes a third part is included as the nasal part of the frontal bone, and sometimes this is included with the squamous part. The nasal part is between the brow ridges, and ends in a serrated nasal notch that articulates with the nasal bones inferiorly, and with the lacrimal and maxilla bones laterally.

Borders

The border of the squamous part is thick, strongly serrated, bevelled at the expense of the inner table above, where it rests upon the parietal bones, and at the expense of the outer table on either side, where it receives the lateral pressure of those bones; this border is continued below into a triangular, rough surface, which articulates with the great wing of the sphenoid. The posterior borders of the orbital plates are thin and serrated, and articulate with the small wings of the sphenoid. [1]

Development

The frontal bone is presumed to be derived from neural crest cells.[2]

The frontal bone is ossified in membrane from two primary centers, one for each half, which appear toward the end of the second month of fetal life, one above each supraorbital margin. From each of these centers, ossification extends upward to form the corresponding half of the squama, and backwards to form the orbital plate. The spine is ossified from a pair of secondary centers, on either side of the middle line; similar centers appear in the nasal part and zygomatic processes.

At birth the bone consists of two pieces, separated by the frontal suture, which is usually obliterated, except at its lower part, by the eighth year, but occasionally persists throughout life. It is generally maintained that the development of the frontal sinuses begins at the end of the first or beginning of the second year, but may begin at birth. The sinuses are of considerable size by the seventh or eighth year, but do not attain their full proportions until after puberty.

Other animals

In most vertebrates, the frontal bone is paired, rather than presenting the single, fused structure found in humans (see frontal suture). It typically lies on the upper part of the head, between the eyes, but in many non-mammalian animals it does not form part of the orbital cavity. Instead, in reptiles, bony fish and amphibians it is often separated from the orbits by one or two additional bones not found in mammals. These bones, the prefrontals and postfrontals, together form the upper margin of the eye sockets, and lie to either side of the frontal bones.[3]

Dinosaurs

The frontal bone is one of the principal paired mid-line bones in dinosaur skulls. This bone is part of the skull roof, which is a set of bones that cover the brain, eyes and nostrils. The frontal makes contact with several other bones in the skull. The anterior part of the bone articulates with the nasal bone and the prefrontal bone. The posterior part of the bone articulates with the postorbital bone and the parietal bone. This bone defines all of part of the upper margin of the orbit.

See also

References

This article incorporates text in the public domain from page 135 of the 20th edition of Gray's Anatomy (1918)

  1. ^ a b Gray's Anatomy (1918)
  2. ^ Kirby, M. L.; Waldo, K. L. (1990). "Role of neural crest in congenital heart disease". Circulation. 82 (2): 332–340. doi:10.1161/01.CIR.82.2.332. PMID 2197017.
  3. ^ Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 226–241. ISBN 0-03-910284-X.

External links

Coronal suture

The coronal suture is a dense, fibrous connective tissue joint that separates the two parietal bones from the frontal bone of the skull.

Dacryon

The point of junction of the maxillary bone, lacrimal bone, and frontal bone is named the dacryon.

Eupelycosauria

The Eupelycosauria originally referred to a suborder of 'pelycosaurs' (Reisz 1987), but has been redefined (Laurin and Reisz 1997) to designate a clade of synapsids that includes most pelycosaurs, as well as all therapsids and mammals. They first appear during the Early Pennsylvanian epoch (i.e.: Archaeothyris, and perhaps an even earlier genus, Protoclepsydrops), and represent just one of the many stages in the acquiring of mammal-like characteristics (Kemp 1982), in contrast to their earlier amniote ancestors. The defining characteristics which separate these animals from the Caseasauria (also pelycosaurs) are based on details of proportion of certain bones of the skull. These include a long, narrow supratemporal bone (in contrast to caseasaurs where this bone is almost as wide as it is long), and a frontal bone with a wider connection to the upper margin of the orbit (Laurin and Reisz 1997).

Foramen cecum (frontal bone)

The frontal crest of the frontal bone ends below in a small notch which is converted into a foramen, the foramen cecum (or foramen caecum), by articulation with the ethmoid.

The foramen cecum varies in size in different subjects, and is frequently impervious; when open, it transmits the emissary vein from the nose to the superior sagittal sinus. This has clinical importance in that infections of the nose and nearby areas can be transmitted to the meninges and brain from what is known as the danger triangle of the face.

Fossa for lacrimal gland

The lacrimal fossa (or fossa for lacrimal gland) is located on the inferior surface of each orbital plate of the frontal bone. It is smooth and concave, and presents, laterally, underneath the zygomatic process, a shallow depression for the lacrimal gland.

Frontal crest

The internal surface of the squamous part of the frontal bone is concave and presents in the upper part of the middle line a vertical groove, the sagittal sulcus, the edges of which unite below to form a ridge, the frontal crest; the sulcus lodges the superior sagittal sinus, while its margins and the crest afford attachment to the falx cerebri.

Frontal eminence

A frontal eminence (or tuber frontale) refers to one of two rounded elevations on the frontal bone that lie about three centimeters above the supraorbital margin on each side of the frontal suture.

Eminences vary in size in different individuals, are occasionally asymmetrical, and are especially prominent in young skulls. The surface of the bone above them is smooth, and covered by the galea aponeurotica.

Frontal scale

Frontal scale refers to the scales of a reptile which lie in the general region of the forehead of a snake, more specifically between the eyes and to the anterior of this area. These are analogous to the frontal bone on a human which corresponds to the forehead.

Snake scales attached to the frontals and to its anterior are called prefrontals.

Frontal suture

The frontal suture is a fibrous joint that divides the two halves of the frontal bone of the skull in infants and children. Typically, it completely fuses between three and nine months of age, with the two halves of the frontal bone being fused together. It is also called the metopic suture, although this term may also refer specifically to a persistent frontal suture.If the suture is not present at birth because both frontal bones have fused (craniosynostosis), it will cause a keel-shaped deformity of the skull called "trigonocephaly".

Its presence in a fetal skull, along with other cranial sutures and fontanelles, provides a malleability to the skull that can facilitate movement of the head through the cervical canal and vagina during delivery. The dense connective tissue found between the frontal bones is replaced with bone tissue as the child grows older.

Frontoethmoidal suture

The frontoethmoidal suture is the suture between the ethmoid bone and the frontal bone.

It is located in the anterior cranial fossa.

Orbital part of frontal bone

The orbital or horizontal part of the frontal bone (pars orbitalis) consists of two thin triangular plates, the orbital plates, which form the vaults of the orbits, and are separated from one another by a median gap, the ethmoidal notch.

Orbital sulcus

The inferior or orbital surface of the frontal lobe is concave, and rests on the orbital plate of the frontal bone. It is divided into four orbital gyri by a well-marked H-shaped orbital sulcus

Sagittal keel

The sagittal keel (torus) is a thickening of bone on part or all of the midline of the frontal bone, or parietal bones where they meet along the sagittal suture, or on both bones. Sagittal keels differ from sagittal crests, which are found in some earlier hominins (notably the genus Paranthropus) and in a range of other mammals. While a proper crest functions in anchoring the muscles of mastication to the cranium, the keel is lower and rounded in cross-section, and the jaw muscles do not attach to it.

Sagittal keels occur in several early human species, most noticeably in Homo erectus, occasionally in Homo heidelbergensis and in some Upper Paleolithic Homo Sapiens specimens. Most modern Homo sapiens groups have lost them, likely as part of the general trend toward thinning of the cranial bones to make room for larger brains during the Pleistocene. However, there is a very small portion of modern humans who have this, but its function and etiology are unknown. Patrick Stewart of Star Trek: The Next Generation and the martial artist Shi Yan Ming present good examples of modern humans (Homo sapiens sapiens) with this feature. Australian Aborigenes also possess it. The keel appears to be tied to general cranial robusticity, and is more common in adult men than in women, and absent in children.

Sagittal sulcus

The internal surface of the squama frontalis of the frontal bone is concave and presents in the upper part of the middle line a vertical groove, the sagittal sulcus, the edges of which unite below to form a ridge, the frontal crest; the sulcus lodges the superior sagittal sinus, while its margins and the crest afford attachment to the falx cerebri.

It also is part of the parietal, and occipital bones.

Sphenofrontal suture

The Sphenofrontal suture is the cranial suture between the sphenoid bone and the frontal bone.

Squamous part of the frontal bone

There are two surfaces of the squamous part of the frontal bone: the external surface, and the internal surface.

Supraorbital foramen

The supraorbital foramen, is a bony elongated opening located above the orbit (eye socket) and under the forehead. The supraorbital foramen lies directly under the eyebrow. Sometimes this foramen is incomplete and is then known as the suprorbital notch.

Trochlear fovea

Near the nasal part of the interior surface of the frontal bone is a depression, the trochlear fovea, or occasionally a small trochlear spine, for the attachment of the cartilaginous pulley of the Obliquus oculi superior.

Zygomatic process

Each Zygomatic process is the part of a bone which articulates with the zygomatic bone. The three processes are:

Zygomatic process of frontal bone from the frontal bone

Zygomatic process of maxilla from the maxilla (malar process)

Zygomatic process of temporal bone from the temporal boneThe term zygomatic derives from the Greek Ζυγόμα zygoma meaning "yoke". The zygomatic process is occasionally referred to as the zygoma, but this term usually refers to the zygomatic bone or occasionally the zygomatic arch.

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