View source for Lip

{{short description|Visible body parts at the mouth}}
{{redirect|Lips|other uses|Lip (disambiguation)}}
{{About|the skin and muscle in front of the mouth|the mouth itself|Human mouth}} {{Not to be confused with|Lisp}}
{{more citations needed|date=September 2015}}
{{Anatomy-terms}}
{{Infobox anatomy
| Name        = Lip
| Latin       = labia oris
| Image       = Lips 2023.jpg
| Caption     = Human lips
| Precursor   =
| System      =
| Artery      = [[Inferior labial artery|Inferior labial]], [[superior labial artery|superior labial]]
| Vein        = [[Inferior labial vein|Inferior labial]], [[superior labial vein|superior labial]]
| Nerve       = [[frontal nerve|Frontal]], [[Infraorbital nerve|infraorbital]]
}}
The '''lips''' are a horizontal pair of [[soft tissue|soft]] [[appendage]]s attached to the [[jaw]]s and are the most visible part of the [[mouth]] of many [[animal]]s, including [[human]]s.<ref>{{Cite web |title=Definition of LIP |url=https://www.merriam-webster.com/dictionary/lip |access-date=2022-05-22 |website=Merriam-Webster |language=en}}</ref> [[Mammal]] lips are soft, movable and serve to facilitate the [[ingestion]] of [[food]] (e.g. [[suckling]] and [[gulping]]) and the [[articulatory phonetics|articulation]] of sound and [[speech]]. Human lips are also a [[somatosensory]] organ, and can be an [[erogenous zone]] when used in [[kiss]]ing and other acts of [[intimacy]].

== Structure ==
[[File:MouthCupidBow.jpg|thumbnail|Cupid's bow feature of a human lip]]
The upper and lower lips are referred to as the ''labium superius oris'' and ''labium inferius oris'', respectively.<ref>{{cite web |url=http://www.medilexicon.com/medicaldictionary.php?t=47448 |title=Labium Superius Oris -- Medical Definition |website=medilexicon.com |access-date=2011-11-12 |archive-url=https://web.archive.org/web/20141017162121/http://www.medilexicon.com/medicaldictionary.php?t=47448 |archive-date=2014-10-17 |url-status=dead }}</ref><ref>{{cite web |url=http://www.medilexicon.com/medicaldictionary.php?t=47434 |title=Labium Inferius Oris -- Medical Definition |website=medilexicon.com |access-date=2011-11-12 |archive-url=https://web.archive.org/web/20141017161949/http://www.medilexicon.com/medicaldictionary.php?t=47434 |archive-date=2014-10-17 |url-status=dead }}</ref>  The juncture where the lips meet the surrounding skin of the mouth area is the [[vermilion border]],<ref>{{cite web |url=http://www.medilexicon.com/medicaldictionary.php?t=11482 |title=Vermilion Border -- Medical Definition |website=medilexicon.com |access-date=2011-11-12 |archive-url=https://web.archive.org/web/20141017162120/http://www.medilexicon.com/medicaldictionary.php?t=11482 |archive-date=2014-10-17 |url-status=dead }}</ref> and the typically reddish area within the borders is called the vermilion zone.<ref>{{cite web |url=http://www.medilexicon.com/medicaldictionary.php?t=100530 |title=Vermilion Zone -- Medical Definition |website=medilexicon.com |access-date=2011-11-12 |archive-url=https://web.archive.org/web/20141017161903/http://www.medilexicon.com/medicaldictionary.php?t=100530 |archive-date=2014-10-17 |url-status=dead }}</ref>  The vermilion border of the upper lip is known as the [[Cupid's bow]].<ref>{{cite web |url=http://www.medilexicon.com/medicaldictionary.php?t=11599 |title=Cupid's Bow -- Medical Definition |website=medilexicon.com |access-date=2011-11-12 |archive-url=https://web.archive.org/web/20141017162205/http://www.medilexicon.com/medicaldictionary.php?t=11599 |archive-date=2014-10-17 |url-status=dead }}</ref>  The fleshy protuberance located in the center of the upper lip is a [[Tubercle (anatomy)|tubercle]] known by various terms including the procheilon (also spelled ''prochilon''), the "tuberculum labii superioris", and the "labial tubercle".<ref>{{cite web |url=http://www.medilexicon.com/medicaldictionary.php?t=94560 |title=Tubercle Of Upper Lip -- Medical Definition |website=medilexicon.com |access-date=2011-11-12 |archive-url=https://web.archive.org/web/20141017162202/http://www.medilexicon.com/medicaldictionary.php?t=94560 |archive-date=2014-10-17 |url-status=dead }}</ref>  The vertical groove extending from the procheilon to the [[nasal septum]] is called the [[philtrum]].<ref>{{cite web |url=http://www.medilexicon.com/medicaldictionary.php?t=68105 |title=Philtrum -- Medical Definition |website=medilexicon.com |access-date=2011-11-12 |archive-url=https://web.archive.org/web/20141017161952/http://www.medilexicon.com/medicaldictionary.php?t=68105 |archive-date=2014-10-17 |url-status=dead }}</ref>
[[File:Anatomy of the human lip.jpg|thumb|Surface anatomy of the human lips|320x320px]]
The skin of the lip, with three to five cellular layers, is very thin compared to typical face skin, which has up to 16 layers. With light skin color, the lip skin contains fewer [[melanocytes]] (cells which produce melanin [[pigment]], which give skin its color). Because of this, the [[blood vessel]]s appear through the skin of the lips, which leads to their notable red coloring. With darker skin color this effect is less prominent, as in this case the skin of the lips contains more melanin and thus is visually darker.  The skin of the lip forms the border between the exterior skin of the face, and the interior [[mucous membrane]] of the inside of the mouth.

The lip skin is not hairy and does not have [[sweat gland]]s. Therefore, it does not have the usual protection layer of sweat and body oils which keep the skin smooth, inhibit pathogens, and regulate warmth. For these reasons, the lips dry out faster and become [[chapped]] more easily.

The lower lip is formed from the [[mandibular prominence]], a branch of the [[first pharyngeal arch]]. The lower lip covers the anterior [[body of the mandible]]. It is lowered by the [[depressor labii inferioris muscle]] and the [[orbicularis oris]] borders it inferiorly.

The upper lip covers the [[anterior surface of the body of the maxilla]]. Its upper half is of usual skin color and has a depression at its center, directly under the nasal septum, called the [[philtrum]], which is Latin for "lower nose", while its lower half is a markedly different, red-colored skin tone more similar to the color of the inside of the mouth, and the term ''vermillion'' refers to the colored portion of either the upper or lower lip.

It is raised by the [[levator labii superioris]] and is connected to the [[lower lip]] by the thin lining of the lip itself.

Thinning of the vermilion of the upper lip and flattening of the [[philtrum]] are two of the facial characteristics of [[fetal alcohol syndrome]], a lifelong [[disability]] caused by the mother's consumption of alcohol during pregnancy.

===Microanatomy ===
The skin of the lips is [[stratified squamous epithelium]]. The mucous membrane is represented by a large area in the [[somatosensory system|sensory cortex]], and is therefore highly sensitive. The ''[[Frenulum of lower lip|frenulum labii inferioris]]'' is the [[frenulum]] of the lower lip. The ''frenulum labii superioris'' is the frenulum of the upper lip.

===Nerve supply===
[[File:Gray509.png|thumb|Illustration of lips from ''[[Gray's Anatomy]]'' showing the [[Inferior labial artery|inferior]] and [[Superior labial artery|superior labial arteries]], the glands of the lips, and the nerves of the right side seen from the posterior surface after removal of the mucous membrane]]
* [[Trigeminal nerve]]
** The [[infraorbital nerve]] is a branch of the [[Maxillary nerve|maxillary]] branch. It supplies not only the upper lip but also much of the skin of the face between the upper lip and the lower eyelid, except for the bridge of the nose.
** The [[mental nerve]] is a branch of the [[Human mandible|mandibular]] branch (via the [[inferior alveolar nerve]]).  It supplies the skin and mucous membrane of the lower lip and labial [[gingiva]] (gum) anteriorly.

=== Blood supply ===
The [[facial artery]] is one of the six non-terminal branches of the [[external carotid artery]].

This artery supplies both lips by its superior and inferior labial branches. Each of the two branches ''bifurcate'' and ''anastomose'' with their companion branch from the other terminal.

=== Muscles ===
The muscles acting on the lips are considered part of the muscles of [[facial expression]].  All muscles of facial expression are derived from the [[mesoderm]] of the second [[pharyngeal arch]] and are therefore supplied (motor supply) by the nerve of the second pharyngeal arch, the [[facial nerve]] (7th [[cranial nerve]]). The muscles of facial expression are all specialized members of the [[panniculus carnosus]], which attach to the [[dermis]] and so wrinkle or dimple the overlying skin. Functionally, the muscles of facial expression are arranged in groups around the [[Orbit (anatomy)|orbits]], nose, and mouth.

The muscles acting on the lips:
* [[buccinator muscle|Buccinator]]
* [[orbicularis oris muscle|Orbicularis oris]] (a complex of muscles, formerly thought to be a single [[sphincter]] or ring of muscle)
* Anchor point for several muscles
** [[modiolus (anatomy)|Modiolus]]
* Lip elevation
** [[Levator labii superioris]]
** [[Levator labii superioris alaeque nasi muscle|levator labii superioris alaeque nasi]]
** [[Levator anguli oris]]
** [[zygomaticus minor muscle|Zygomaticus minor]]
** [[zygomaticus major muscle|Zygomaticus major]]
* Lip depression
** [[Risorius]]
** [[depressor anguli oris muscle|Depressor anguli oris]]
** [[depressor labii inferioris muscle|Depressor labii inferioris]]
** [[Mentalis]]

== Functions ==

=== Food intake ===
[[File:Breastfeeding infant.jpg|thumb|upright|[[Breastfeeding]]]]
Because they have their own muscles and bordering muscles, the lips are easily movable. Lips are used for eating functions, like holding food or to get it in the mouth. In addition, lips serve to close the mouth airtight shut, to hold food and drink inside, and to keep out unwanted objects. Through making a narrow [[funnel]] with the lips, the suction of the mouth is increased. This suction is essential for babies to [[breast feed]]. Lips can also be used to suck in other contexts, such as sucking on a straw to drink liquids.

=== Articulation ===
The lips serve for creating different sounds—mainly [[labial consonant|labial]], [[bilabial consonant|bilabial]], and [[labiodental consonant|labiodental]] consonant sounds as well as [[roundedness|vowel rounding]]—and thus are an important part of the speech apparatus.  The lips enable [[whistling]] and the performing of wind instruments such as the trumpet, [[clarinet]], flute, and saxophone. People who have [[hearing loss]] may unconsciously or consciously [[lip reading|lip read]] to understand speech without needing to perceive the actual sounds, and visual cues from the lips affect the perception of what sounds have been heard, for example the [[McGurk effect]].

=== Tactile organ ===
The lip has many [[nerve ending]]s  and reacts as part of the [[somatosensory system|tactile]] (touch) senses.  Lips are very sensitive to touch, warmth, and cold.  It is therefore an important aid for exploring unknown objects for babies and toddlers.

=== Erogenous zone ===
[[File:Lujuria Lust Tentación (3973672868).jpg|thumb|upright|Lips of a young woman wearing red [[lipstick]]]]
[[File:Close up man lips.jpg|thumb|upright|Lips of a young [[man]]]]
Because of their high number of nerve endings, the lips are an [[erogenous zone]]. The lips therefore play a crucial role in [[kissing]] and other acts of intimacy.

A woman's lips are also a visible expression of her fertility. In studies performed on the science of human attraction, psychologists have concluded that a woman's facial and sexual attractiveness is closely linked to the makeup of her hormones during puberty and development.  Contrary to the effects of [[testosterone]] on a man's facial structure, the effects of a woman's [[oestrogen]] levels serve to maintain a relatively "childlike" and youthful facial structure during puberty and during final maturation.  It has been shown that the more oestrogen a woman has, the larger her eyes and the fuller her lips, characteristics which are perceived as more feminine.<ref>{{cite journal |first1=Miriam J. |last1=Law Smith |first2=Denis K. |last2=Deady |first3=Fhionna R. |last3=Moore |first4=Benedict C. |last4=Jones |first5=R. Elisabeth |last5=Cornwell |first6=Michael |last6=Stirrat |first7=Jamie F. |last7=Lawson |first8=David R. |last8=Feinberg |first9=David I. |last9=Perrett |title=Maternal tendencies in women are associated with estrogen levels and facial femininity |journal=Hormones and Behavior |volume=61 |issue=1 |date=2011-09-21 |doi=10.1016/j.yhbeh.2011.09.005 |pages=12–6 |pmid=21983237|s2cid=23542024}}</ref>  Surveys performed by sexual psychologists have also found that universally, men find a woman's full lips to be more sexually attractive than lips that are less so. A woman's lips are therefore sexually attractive to males because they serve as a biological indicator of a woman's health and fertility.  A woman's lipstick (or [[collagen]] [[lip enhancement]]) attempts to take advantage of this fact by creating the illusion that a woman has more oestrogen than she actually has and thus that she is more fertile and attractive.<ref name="titleWhy do men find big lips and little noses so sexy? I'll paint you a picture – Comment – Times Online">{{cite news |archive-url=https://web.archive.org/web/20070222220710/http://www.timesonline.co.uk/tol/comment/columnists/guest_contributors/article597433.ece |archive-date=February 22, 2007 |url=http://www.timesonline.co.uk/article/0,,1072-1894256,00.html |title=Why do men find big lips and little noses so sexy? I'll paint you a picture – Comment – Times Online |access-date=2007-12-12 |work=The Times |location=London |first=Science |last=Note |date=2005-11-28}}</ref>

Lip size is linked to sexual attraction in both men and women. Women are attracted to men with masculine lips that are more middle size and not too big or too small; they are to be rugged and sensual. In general, the researchers found that a small nose, big eyes and voluptuous lips are sexually attractive both in men and women.<ref name="Lip Size Key To Sexual Attraction">{{cite news |url=http://news.bbc.co.uk/2/hi/health/2817795.stm |title=Lip size key to sexual attraction |access-date=2010-01-15 |publisher=[[BBC News]] |date=2003-03-04}}</ref> The lips may temporarily swell during sexual arousal due to engorgement with blood.{{medcn|date=January 2015}}

=== Facial expression ===
The lips contribute substantially to [[facial expression]]s. The lips visibly express emotions such as a [[smile]] or frown, iconically by the curve of the lips forming an up-open or down-open arc, respectively. Lips can also be made pouty when whining or perky to be provocative.

=== Open questions ===
The function of the abrupt change in skin structure between the lips and surrounding face (in particular, the function of the less keratinized [[vermillion border|vermillion]] and the [[white roll]]) is not completely understood.<ref>{{Cite journal|last1=Kobayashi |first1=H|last2=Tagami |first2=H|date=March 2004|url=https://pubmed.ncbi.nlm.nih.gov/15030342/|title=Functional properties of the surface of the vermilion border of the lips are distinct from those of the facial skin.|pmid=15030342|journal=British Journal of Dermatology|volume=150|issue=3|pages=563–567|doi=10.1046/j.1365-2133.2003.05741.x|s2cid=42980307}}</ref> Possible reasons for the difference may include advantages to somatosensory function, better communication of facial expressions, and/or emphasis of the lips' slight [[sexual dimorphism]] as a [[secondary sex characteristic]].

== Clinical significance==
{{unreferenced section|date=September 2015}}
As an organ of the body, the lip can be a focus of disease or show symptoms of a disease:
* One of the most frequent changes of the lips is a blue coloring due to [[cyanosis]]; the blood contains less oxygen and thus has a dark red to blue color, which shows through the thin skin.  Cyanosis is the reason why corpses sometimes have blue lips. In cold weather cyanosis can appear, so especially in the winter, blue lips may not be an uncommon sight.<ref>{{Citation|last1=Adeyinka|first1=Adebayo|title=Cyanosis|date=2021|url=http://www.ncbi.nlm.nih.gov/books/NBK482247/|work=StatPearls|place=Treasure Island (FL)|publisher=StatPearls Publishing|pmid=29489181|access-date=2021-07-08|last2=Kondamudi|first2=Noah P.}}</ref>
* Inflammation of the lips is termed [[cheilitis]]. This can be in several forms such as [[chapped lips]] (dry, peeling lips), [[angular cheilitis]] (inflammation of the corners of the mouth), [[herpes labialis]] (cold sore, a form of [[herpes simplex]]) and [[actinic cheilitis]] (chronically sun damaged lips).<ref>{{Citation|last1=Bhutta|first1=Beenish S.|title=Cheilitis|date=2021|url=http://www.ncbi.nlm.nih.gov/books/NBK470592/|work=StatPearls|place=Treasure Island (FL)|publisher=StatPearls Publishing|pmid=29262127|access-date=2021-07-08|last2=Hafsi|first2=Wissem}}</ref>
[[File:Cleft lip child.jpg|thumb|A child with [[cleft lip]]]]
* [[Cleft lip]] is a type of [[birth defect]] that can be successfully treated with surgery.<ref>{{Cite web|last=CDC|date=2020-12-28|title=Facts about Cleft Lip and Cleft Palate {{!}} CDC|url=https://www.cdc.gov/ncbddd/birthdefects/cleftlip.html|access-date=2021-07-08|website=Centers for Disease Control and Prevention|language=en-us}}</ref>
* [[Carcinoma]] (a malignant cancer that arises from epithelial cells) at the lips is caused predominantly by using tobacco and overexposure of sunlight.<ref>{{Cite web|title=Lip cancer - Symptoms and causes|url=https://www.mayoclinic.org/diseases-conditions/lip-cancer/symptoms-causes/syc-20355079|access-date=2021-07-08|website=Mayo Clinic|language=en}}</ref> Alcohol appears to increase the carcinoma risk associated with tobacco use.<ref>{{Cite web|title=Risk Factors for Oral Cavity and Oropharyngeal Cancers|url=https://www.cancer.org/cancer/oral-cavity-and-oropharyngeal-cancer/causes-risks-prevention/risk-factors.html|access-date=2021-07-08|website=www.cancer.org|language=en}}</ref> It is most often a diffuse and often hyperkeratinised lesion, occasionally has the form of nodules and grows infiltratively, and can also be a combination of the two types. It more often occurs at the lower lip, where it is also much more malign. Lower lip carcinoma is exclusively [[planocellular carcinoma]], whereas at the upper lip, it can also be [[basocellular carcinoma]].<ref>{{Cite journal|last1=MORETTI|first1=A.|last2=VITULLO|first2=F.|last3=AUGURIO|first3=A.|last4=PACELLA|first4=A.|last5=CROCE|first5=A.|date=2011|title=Surgical management of lip cancer|journal=Acta Otorhinolaryngologica Italica|volume=31|issue=1|pages=5–10|issn=0392-100X|pmc=3146335|pmid=21808457}}</ref>

== Society and culture ==
[[File:Sparkly lips.jpg|thumb|upright=0.6|[[Lip piercing|Pierced lips]]]]
Lips are often viewed as a symbol of sensuality and sexuality. This has many origins; above all, the lips are a very sensitive erogenous and tactile organ. Furthermore, in many cultures of the world, a woman's mouth and lips are veiled because of their representative association with the vulva, and because of their role as a woman's secondary sexual organ.<ref name="Mouth veiled">{{cite book |author=Valsiner, Jaan |title=Culture and Human Development |publisher=Sage Publications, Ltd |year=2000 |pages=134–136}}</ref>

As part of the mouth, the lips are also associated with the symbolism associated with the mouth as orifice by which food is taken in.  The lips are also linked symbolically to neonatal psychology (see for example [[oral stage]] of the psychology according to [[Sigmund Freud]]).

[[Lip piercing]] or [[lip augmentation]] is sometimes carried out for cosmetic reasons. Products designed for use on the lips include lipstick, [[lip gloss]] and [[lip balm]].

==Other animals ==
[[File:Arowana.jpg|thumb|This [[Asian arowana]] has large, protruding [[barbel (anatomy)|barbel]]s]]
In most vertebrates, the lips are relatively unimportant folds of tissue lying just outside the jaws. However, in [[mammal]]s, they become much more prominent, being separated from the jaws by a deep cleft{{Citation needed|reason=source in citation at end of sentence is for naked mole rats having lips close behind their front teeth, there is no source for "most vertebrates' lips being basically unimportant folds of tissue just outside the jaws" or for "mammals' lips are separated from the jaws by a deep cleft" |date=November 2020}} (a notable exception being the [[naked mole-rat]], whose lips close behind the front teeth).<ref>{{cite journal |last1=Griffin |first1=Ashleigh S. |title=Naked Mole Rat |journal=Current Biology |date=23 September 2008 |volume=18 |issue=18 |pages=R844-5 |doi=10.1016/j.cub.2008.07.054 |pmid=18812073 |bibcode=2008CBio...18.R844G |s2cid=42179436 |url=https://www.cell.com/current-biology/pdf/S0960-9822(08)00963-9.pdf |access-date=9 November 2020}}</ref> They are also more mobile in mammals than in other groups since it is only in this group that they have any attached muscles. In some [[teleost]] fish, the lips may be modified to carry sensitive [[barbel (anatomy)|barbels]]. In birds and turtles, the lips are hard and [[keratin]]ous, forming a solid [[beak]].<ref name=VB>{{cite book |author=Romer, Alfred Sherwood |author2=Parsons, Thomas S. |year=1977 |title=The Vertebrate Body |publisher=[[Holt-Saunders International]] |location=Philadelphia|pages=297 |isbn=978-0-03-910284-5}}</ref> [[Clevosauridae|Clevosaurids]] like ''[[Clevosaurus]]'' are notable for the presence of bone "lips"; in these species the tooth-like jaw projections common to all [[sphenodontia]]ns form a beak-like edge around the jaws, protecting the teeth within.<ref>Jones MEH (2009). "Dentary tooth shape in Sphenodon and its fossil relatives (Diapsida: Lepidosauria: Rhynchocephalia)". In Koppe T, Meyer G, Alt KW, (eds). ''Interdisciplinary Dental Morphology, Frontiers of Oral Biology'' (vol 13). Griefswald, Germany; [[Karger]]. 9–15.</ref>

== See also ==
* [[Stiff upper lip]]

== References ==
{{Reflist}}

== Further reading ==
* {{cite journal |vauthors=Tomiyama N, Ichida T, Yamaguchi K |title=Electromyographic activity of lower lip muscles when chewing with the lips in contact and apart |journal=Angle Orthod |volume=74 |issue=1 |pages=31–6 |year=2004 |pmid=15038488 |url=http://journals.allenpress.com/jrnlserv/?request=get-abstract&issn=0003-3219&volume=74&page=31 |url-access=subscription |type=Abstract}}
* {{cite journal |vauthors=Bisson M, Grobbelaar A |title=The esthetic properties of lips: a comparison of models and nonmodels |journal=Angle Orthod |volume=74 |issue=2 |pages=162–6 |year=2004 |pmid=15132441 |url=http://journals.allenpress.com/jrnlserv/?request=get-abstract&issn=0003-3219&volume=74&page=162 |url-access=subscription |type=Abstract}}
* {{cite book |author1=McMinn, R. M. H. |author2=Last, R. J. |title=Last's anatomy, regional and applied |publisher=[[Churchill Livingstone]] |location=Edinburgh |year=1994 |isbn=978-0-443-04662-9 }}

== External links ==
* {{eMedicine|ent|7}}
* {{EmbryologyUNC|hednk|030}}
* [https://web.archive.org/web/20061230085921/http://oralhealth.dent.umich.edu/VODI/html/03-oc/a-anatomy/lips.html Anatomy at oralhealth.dent.umich.edu]
* {{Commons cat inline|Lips}}
* {{Wikiquote inline|Lips}}

{{Human regional anatomy}}
{{Human systems and organs}}
{{Mouth anatomy}}
{{Authority control}}

[[Category:Facial features]]
[[Category:Mouth]]
[[Category:Human anatomy|Lips]]
[[Category:Human head and neck]]
[[Category:Speech organs]]
[[Category:Human mouth anatomy]]
[[Category:Digestive system]]
[[Category:Lips|*]]