![Crane3chin](https://upload.wikimedia.org/wikipedia/commons/0/06/Crane3chin.png)
The chin is the forward pointed part of the anterior
mandible
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
(
mental region) below the lower
lip. A fully developed human skull has a chin of between 0.7 cm and 1.1 cm.
Evolution
The presence of a well-developed chin is considered to be one of the morphological characteristics of ''
Homo sapiens
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
'' that
differentiates them from other human ancestors such as the closely related
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 ...
. Early human ancestors have varied
symphysial morphology, but none of them have a well-developed chin. The origin of the chin is traditionally associated with the anterior–posterior breadth shortening of the
dental arch
The dental arches are the two arches (crescent arrangements) of teeth, one on each jaw, that together constitute the dentition. In humans and many other species; the superior (maxillary or upper) dental arch is a little larger than the inferio ...
or tooth row; however, its general mechanical or functional advantage during feeding, developmental origin, and link with human speech, physiology, and social influence are highly debated.
Functional perspectives
Robinson (1913) suggests that the demand to resist
masticatory
Chewing or mastication is the process by which food is 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 process, th ...
stresses triggered bone thickening in the mental region of the
mandible
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
and ultimately formed a prominent chin. Moreover, Daegling (1993) explains the chin as a functional adaptation to resist masticatory stress that causes vertical bending stresses in the
coronal plane
The coronal plane (also known as the frontal plane) is an anatomical plane that divides the body into dorsal and ventral sections. It is perpendicular to the sagittal and transverse planes.
Details
The coronal plane is an example of a longitud ...
. Others have argued that the prominent chin is adapted to resisting wishboning forces, dorso-ventral shear forces, and generally a mechanical advantage to resist lateral transverse bending and vertical bending in the coronal plane. On the contrary, others have suggested that the presence of the chin is not related to mastication. The presence of thick bone in the relatively small mandible may indicate better force resistance capacity. However, the question stands of whether the chin is an
adaptive or nonadaptive structure.
Developmental perspectives
Recent works on the
morphological changes of the mandible during development
have shown that the human chin, or at least the inverted-T shaped mental region, develops during the
prenatal period, but the chin does not become prominent until the early
postnatal period. This later modification happens by
bone remodeling processes (
bone resorption and
bone deposition).
Coquerelle et al.
show that the anteriorly positioned
cervical column of the spine and forward displacement of the
hyoid bone
The hyoid bone (lingual bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies between the base of the mandible and the third cervical vertebr ...
limit the anterior–posterior breadth in the oral cavity for the tongue,
laryngeal, and
suprahyoid
The suprahyoid muscles are four muscles located above the hyoid bone in the neck. They are the digastric, stylohyoid, geniohyoid, and mylohyoid muscles. They are all pharyngeal muscles, with the exception of the geniohyoid muscle. The digastric ...
musculatures. Accordingly, this leads the upper parts of the mandible (
alveolar process
The alveolar process () or alveolar bone is the thickened ridge of bone that contains the tooth sockets on the jaw bones (in humans, the maxilla and the mandible). The structures are covered by gums as part of the oral cavity.
The synonymous ter ...
) to retract posteriorly, following the posterior movement of the upper tooth row, while the lower part of the symphysis remained protruded to create more space, thereby creating the inverted-T shaped mental relief during early ages and the prominent chin later. The alveolar region (upper or superior part of the symphysis) is sculpted by bone resorption, but the chin (lower or inferior part) is depository in its nature.
These coordinated bone growth and modeling processes mold the vertical symphysis present at birth into the prominent shape of the chin.
Recent research on the development of the chin
suggests that the evolution of this unique characteristic was formed not by mechanical forces such as chewing but by evolutionary adaptations involving reduction in size and change in shape of the face. Holton et al. claim that this adaptation occurred as the face became smaller compared to that of other ancient humans.
Other perspectives
Robert Franciscus takes a more anthropological viewpoint: he believes that the chin was formed as a consequence of the change in lifestyle humans underwent approximately 80,000 years ago. As humans'
hunter-gatherer
A traditional hunter-gatherer or forager is a human living an ancestrally derived lifestyle in which most or all food is obtained by foraging, that is, by gathering food from local sources, especially edible wild plants but also insects, fungi, ...
societies grew into larger social networks, territorial disputes decreased because the new social structure promoted building alliances in order to exchange goods and belief systems. Franciscus believes that this change in the human environment reduced hormone levels, especially in men, resulting in the natural evolution of the chin.
Overall, human beings are unique in the sense that they are the only species among
primates
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
who have chins. In the paper ''The Enduring Puzzle of the Human Chin'', evolutionary anthropologists James Pampush and David Daegling discuss various theories that have been raised to solve the puzzle of the chin. They conclude that "each of the proposals we have discussed falter either empirically or theoretically; some fail, to a degree, on both accounts… This should serve as motivation, not discouragement, for researchers to continue investigating this modern human peculiarity… perhaps understanding the chin will reveal some unexpected insight into what it means to be human."
Cleft chin
![Human jawbone front](https://upload.wikimedia.org/wikipedia/commons/d/d5/Human_jawbone_front.jpg)
The terms cleft chin,
chin cleft,
dimple chin, or chin dimple
refer to a
dimple
A dimple, also called a gelasin (, ) is a small natural indentation in the flesh on a part of the human body, most notably in the cheek. Numerous cultures believe that cheek dimples are a good luck charm that entices people who perceive them as ...
on the chin. It is a
Y-shaped fissure on the chin with an underlying bony peculiarity. Specifically, the chin fissure follows the fissure in the lower
jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...
bone that resulted from the incomplete fusion of the left and right halves of the jaw bone, or muscle, during the embryonal and fetal development. It can also develop during the later
mandibular symphysis
In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: ''symphysis menti'') or line of junction where the two lateral halves ...
, due to growth of the mental protuberance during puberty, or as a result of
acromegaly
Acromegaly is a disorder that results from excess growth hormone (GH) after the growth plates have closed. The initial symptom is typically enlargement of the hands and feet. There may also be an enlargement of the forehead, jaw, and nose. Other ...
. In some cases, one
mental tubercle
The mandibular symphysis divides below and encloses a triangular eminence, the mental protuberance
The symphysis of the external surface of the mandible divides below and encloses a triangular eminence, the mental protuberance, the base of which ...
may grow more than another, which can cause facial asymmetry.
[Mammalian Phenotype Browser: Cleft chin]
/ref>
A cleft chin is an inherited trait in humans and can be influenced by many factors. The cleft chin is also a classic example of variable penetrance with environmental factors or a modifier gene
Epistasis is a phenomenon in genetics in which the effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes, respectively termed modifier genes. In other words, the effect of the mutation is dep ...
possibly affecting the phenotypical expression of the actual genotype
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
. Cleft chins can be presented in a child when neither parent presents a cleft chin. Cleft chins are common among people originating from Europe, the Middle East and South Asia.
There is a possible genetic cause for cleft chins, a genetic marker called rs11684042, which is located in chromosome 2.
In Persian literature
Persian literature ( fa, ادبیات فارسی, Adabiyâte fârsi, ) comprises oral compositions and written texts in the Persian language and is one of the world's oldest literatures. It spans over two-and-a-half millennia. Its sources h ...
, the chin dimple is considered a factor of beauty and is metaphorically referred to as "the chin pit" or "the chin well": a well in which the poor lover is fallen and trapped.
Double chin
A double chin is a loss of definition of the jawbone or soft tissue under the chin. There are two possible causes for a double chin, which have to be differentiated.
In overweight people, commonly the layer of subcutaneous fat around the neck
The neck is the part of the body on many vertebrates that connects the head with the torso. The neck supports the weight of the head and protects the nerves that carry sensory and motor information from the brain down to the rest of the body. In ...
sags down and creates a wrinkle
A wrinkle, also known as a rhytid, is a fold, ridge or crease in an otherwise smooth surface, such as on skin or fabric. Skin wrinkles typically appear as a result of ageing processes such as glycation, habitual sleeping positions, loss of bo ...
, creating the appearance of a second chin. This fat pad is occasionally surgically
Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical specialty that uses operative manual and instrumental techniques on a person to investigate or treat a pat ...
removed and the corresponding muscles under the jaw shortened (hyoid lift).
Another cause can be a bony deficiency, commonly seen in people of normal weight. When the jaw bones (mandible
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
and by extension the 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 t ...
) do not project forward enough, the chin in turn will not project forward enough to give the impression of a defined jawline and chin. Despite low amounts of fat in the area, it can appear as if the chin is melting into the neck. The extent of this deficiency can vary drastically and usually has to be treated surgically. In some patients, the aesthetic deficit can be overcome with genioplasty
Chin augmentation using surgical implants can alter the underlying structure of the face, providing better balance to the facial features. The specific medical terms mentoplasty and genioplasty are used to refer to the reduction and addition of ma ...
alone, in others the lack of forward growth might warrant orthognathic surgery to move one or two jaws forward. If the patient suffers from sleep apnea, early maxillomandibular advancement
Maxillomandibular advancement (MMA) or orthognathic surgery, also sometimes called bimaxillary advancement (Bi-Max), or maxillomandibular osteotomy (MMO), is a surgical procedure or sleep surgery which moves the upper jaw (maxilla) and the lower j ...
is usually the only causal treatment and necessary to preserve normal life expectancy.
See also
* Chin augmentation (genioplasty)
* Masseter
* Mastication
Chewing or mastication is the process by which food is 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 process, th ...
* Otofacial syndrome
* Ptosis (chin) A chin that droops over the jawline is called a ptotic chin. Ptosis of the tip of the chin is common and can be seen in persons of any age. It is frequently seen in older patients but not infrequently, it is seen in young people as a familial trait. ...
References
External links
*
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Human head and neck
Facial features