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The brow ridge, or supraorbital ridge known as superciliary arch in medicine, is a bony ridge located above the eye sockets of all
primate Primates are a diverse order (biology), order of mammals. They are divided into the Strepsirrhini, strepsirrhines, which include the lemurs, galagos, and lorisids, and the Haplorhini, haplorhines, which include the Tarsiiformes, tarsiers and ...
s. In
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedality, bipedalism and exceptional cognitive skills due to a large and complex Human brain, brain. This has enabled the development of ad ...
s, the eyebrows are located on their lower margin.


Structure

The brow ridge is a nodule or crest of bone situated on the
frontal bone The frontal bone is a bone in the human skull. The bone consists of two portions.''Gray's Anatomy'' (1918) These are the vertically oriented squamous part of the frontal bone, squamous part, and the horizontally oriented orbital part of frontal bo ...
of the
skull The skull is a bone protective Cranial cavity, cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible ...
. It forms the separation between the forehead portion itself (the
squama frontalis The squamous part of the frontal bone is the superior (approximately two thirds) portion when viewed in standard anatomical orientation. There are two surfaces of the squamous part of the frontal bone: the external surface, and the internal surfac ...
) and the roof of the eye sockets (the pars orbitalis). Normally, in humans, the ridges arch over each eye, offering mechanical protection. In other primates, the ridge is usually continuous and often straight rather than arched. The ridges are separated from the frontal eminences by a shallow groove. The ridges are most prominent medially, and are joined to one another by a smooth elevation named the
glabella The glabella, in humans, is the area of skin between the eyebrows and above the Human nose, nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric analysis#Cephalometric ...
. Typically, the arches are more prominent in men than in women, and vary between different ethnic groups. Behind the ridges, deeper in the bone, are the
frontal sinuses The frontal sinuses are one of the four pairs of paranasal sinuses Paranasal sinuses are a group of four paired skeletal pneumaticity, air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the human eye, eyes; ...
.


Terminology

The brow ridges, being a prominent part of the face in some ethnic groups and a trait linked to both
atavism In biology, an atavism is a modification of a biological structure whereby an ancestral genetic Trait (biological), trait reappears after having been lost through evolutionary change in previous generations. Atavisms can occur in several ways; o ...
and
sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different Morphology (biology), morphological characteristics, particularly characteristics not directly involved in reproduction. The conditio ...
, have a number of names in different disciplines. In vernacular English, the terms eyebrow bone or eyebrow ridge are common. The more technical terms frontal or supraorbital arch, ridge or torus (or tori to refer to the plural, as the ridge is usually seen as a pair) are often found in anthropological or archaeological studies. In medicine, the term ''arcus superciliaris'' (
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
) or the English translation superciliary arch. This feature is different from the supraorbital margin and the margin of the orbit. Some paleoanthropologists distinguish between frontal torus and supraorbital ridge. In anatomy, a ''
torus In geometry, a torus (plural tori, colloquially donut or doughnut) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis that is coplanarity, coplanar with the circle. If the axis of revolution ...
'' is a projecting shelf of bone that unlike a ridge is rectilinear, unbroken and goes through
glabella The glabella, in humans, is the area of skin between the eyebrows and above the Human nose, nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric analysis#Cephalometric ...
. Some fossil
hominin The Hominini form a Tribe (biology), taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and ''Pan (genus), Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus '' ...
s, in this use of the word, have the ''frontal torus'', but almost all modern humans only have the ridge.


Development


Spatial model

The Spatial model proposes that supraorbital torus development can be best explained in terms of the disparity between the anterior position of the orbital component relative the neurocranium. Much of the groundwork for the spatial model was laid down by Schultz (1940). He was the first to document that at later stages of development (after age 4) the growth of the orbit would outpace that of the eye. Consequently, he proposed that facial size is the most influential factor in orbital development, with orbital growth being only secondarily affected by size and ocular position. Weindenreich (1941) and Biegert (1957, 1963) argued that the supraorbital region can best be understood as a product of the orientation of its two components, the face and the neurocranium. The most composed articulation of the spatial model was presented by Moss and Young (1960), who stated that "the presence… of supraorbital ridges is only the reflection of the spatial relationship between two functionally unrelated cephalic components, the orbit and the brain" (Moss and Young, 1960, p282). They proposed (as first articulated by Biegert in 1957) that during infancy the neurocranium extensively overlaps the orbit, a condition that prohibits brow ridge development. As the splanchocranium grows, however, the orbits begin to advance, thus causing the anterior displacement of the face relative to the brain. Brow ridges then form as a result of this separation.


Bio-mechanical model

The bio-mechanical model predicts that morphological variation in torus size is the direct product of differential tension caused by mastication, as indicated by an increase in load/lever ratio and broad craniofacial angle. Research done on this model has largely been based on earlier work of Endo. By applying pressure similar to the type associated with
chewing Chewing or mastication is the process by which food is comminution, crushed and ground by teeth. It is the first step of digestion, and it increases the surface area of foods to allow a more efficient break down by enzymes. During the mastication ...
, he carried out an analysis of the structural function of the supraorbital region on dry human and gorilla
skull The skull is a bone protective Cranial cavity, cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible ...
s. His findings indicated that the face acts as a pillar that carries and disperses tension caused by the forces produced during mastication. Russell and Oyen ''et al''. elaborated on this idea, suggesting that amplified facial projection necessitates the application of enhanced force to the anterior dentition in order to generate the same bite power that individuals with a dorsal deflection of the facial skull exert. In more prognathic individuals, this increased pressure triggers bone deposition to reinforce the brow ridges, until equilibrium is reached. Oyen et al. conducted a cross-section study of ''
Papio anubis The olive baboon (''Papio anubis''), also called the Anubis baboon, is a member of the family Cercopithecidae Old World monkey Old World monkey is the common English name for a family (biology), family of primates known wikt:taxonomy, taxon ...
'' in order to ascertain the relationship between palate length, incisor load and Masseter lever efficiency, relative to torus enlargement. Indications found of osteoblastic deposition in the
glabella The glabella, in humans, is the area of skin between the eyebrows and above the Human nose, nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric analysis#Cephalometric ...
were used as evidence for supraorbital enlargement. Oyen et al.’s data suggested that more prognathic individuals experienced a decrease in load/lever efficiency. This transmits tension via the frontal process of the maxilla to the supraorbital region, resulting in a contemporary reinforcement of this structure. This was also correlated to periods of tooth eruption. In a later series of papers, Russell developed aspects of this mode further. Employing an adult Australian sample, she tested the association between brow ridge formation and anterior dental loading, via the craniofacial angle (prosthion-nasion-metopion),
maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The tw ...
breadth, and discontinuities in food preparation such as those observed between different age groups. Finding strong support for the first two criteria, she concluded that the supraorbital complex is formed as a result of increased tension due to the widening of the maxilla, thought to be positively correlated with the size of the
masseter muscle In human anatomy, the masseter is one of the muscles of mastication There are four classical muscles of mastication. During mastication, three muscles of mastication (''musculi masticatorii'') are responsible for adduction of the jaw, and one ...
, as well as with the improper orientation of bone in the superior orbital region.


Function

Some researchers have suggested that brow ridges function to protect the eyes and orbital bones during hand-to-hand combat, given that they are an incredibly dimorphic trait.


Paleolithic humans

Pronounced brow ridges were a common feature among paleolithic humans. Early modern people such as those from the finds from
Jebel Irhoud Jebel Irhoud or Adrar n Ighoud ( zgh, ⴰⴷⵔⴰⵔ ⵏ ⵉⵖⵓⴷ, Adrar n Iɣud; ar, جبل إيغود, žbəl iġud), is an archaeological site An archaeological site is a place (or group of physical sites) in which evidence of past ...
and Skhul and Qafzeh had thick, large brow ridges, but they differ from those of
archaic humans A number of varieties of ''Homo'' are grouped into the broad category of archaic humans in the period that precedes and is contemporary to the emergence of the earliest early modern humans (''Homo sapiens'') around 300 Year#SI prefix multipliers, ...
like
Neanderthals Neanderthals (, also ''Homo neanderthalensis'' and erroneously ''Homo sapiens neanderthalensis''), also written as Neandertals, are an Extinction, extinct species or subspecies of archaic humans who lived in Eurasia until about 40,000 years ag ...
by having a
supraorbital foramen The supraorbital foramen, is a bony elongated opening located above the orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite a ...
or notch, forming a groove through the ridge above each eye, although there were exceptions, such as Skhul 2 in which the ridge was unbroken, unlike other members of her tribe. This splits the ridge into central parts and distal parts. In current humans, almost always only the central sections of the ridge are preserved (if preserved at all). This contrasts with many archaic and early modern humans, where the brow ridge is pronounced and unbroken.


Other animals

The size of these ridges varies also between different species of primates, either living or fossil. The closest living relatives of
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedality, bipedalism and exceptional cognitive skills due to a large and complex Human brain, brain. This has enabled the development of ad ...
s, the
great apes The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic Family (biology), family of primates that includes eight Neontology#Extant taxa versus extinct taxa, extant species in four Genus, genera: ''Orangutan, ...
and especially
gorilla Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or fi ...
s or
chimpanzee The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa Although tropical Africa is mostly familiar to the Western world, West for its rainfore ...
s, have a very pronounced supraorbital ridge, which has also been called a ''frontal torus'' while in modern humans and
orangutan Orangutans are Hominidae, great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in ...
s, it is relatively reduced. The fossil record indicates that the supraorbital ridge in early hominins was reduced as the cranial vault grew; the frontal portion of the brain became positioned above rather than behind the eyes, giving a more vertical forehead.


See also

* Occipitofrontalis muscle * Supraciliary scale * Frontal sinus


References


Further reading

* * * * *


External links


The Frontal Bone
California State University at Chico site. {{DEFAULTSORT:Supraorbital Ridge Primate anatomy Bones of the head and neck Facial features