The lips are a horizontal pair of soft

appendage An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

s attached to the jaws and are the most visible part of the

mouth A mouth also referred to as the oral is the body orifice through which many animals ingest food and animal communication#Auditory, vocalize. The body cavity immediately behind the mouth opening, known as the oral cavity (or in Latin), is also t ...

of many

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

s, including

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

Mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

lips are soft, movable and serve to facilitate the ingestion of

food Food is any substance consumed by an organism for Nutrient, nutritional support. Food is usually of plant, animal, or Fungus, fungal origin and contains essential nutrients such as carbohydrates, fats, protein (nutrient), proteins, vitamins, ...

(e.g. suckling and gulping) and the articulation of sound and

speech Speech is the use of the human voice as a medium for language. Spoken language combines vowel and consonant sounds to form units of meaning like words, which belong to a language's lexicon. There are many different intentional speech acts, suc ...

. Human lips are also a

somatosensory The somatosensory system, or somatic sensory system is a subset of the sensory nervous system. The main functions of the somatosensory system are the perception of external stimuli, the perception of internal stimuli, and the regulation of bod ...

organ, and can be an

erogenous zone An erogenous zone (from Greek , ''érōs'' "love"; and English ''-genous'' "producing", from Greek , ''-genḗs'' "born") is an area of the human body that has heightened Sensory processing, sensitivity, the sexual stimulation, stimulation of wh ...

when used in

kiss A kiss is the touching or pressing of one's lips against another person, animal or object. Cultural connotations of kissing vary widely; depending on the culture and context, a kiss can express sentiments of love, passion, romance, sex ...

ing and other acts of

intimacy An intimate relationship is an interpersonal relationship that involves emotional or physical closeness between people and may include sexual intimacy and feelings of romance or love. Intimate relationships are interdependent, and the member ...

Structure

The upper and lower lips are referred to as the ''labium superius oris'' and ''labium inferius oris'', respectively. The juncture where the lips meet the surrounding skin of the mouth area is the

vermilion border The vermilion border (sometimes spelled vermillion border), also called margin or zone, is the normally sharp demarcation between the lip and the adjacent normal skin. It represents the change in the epidermis from highly keratinized external ski ...

, and the typically reddish area within the borders is called the vermilion zone. The vermilion border of the upper lip is known as the

Cupid's bow The Cupid's bow is a facial feature where the double curve of a human upper lip is said to resemble a recurve bow of the sort used in ancient Greece or Rome. The name is taken from Cupid, the bow-wielding Roman god of erotic love equivalent to ...

. The fleshy protuberance located in the center of the upper lip is a

tubercle In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projectio ...

known by various terms including the procheilon (also spelled ''prochilon''), the "tuberculum labii superioris", and the "labial tubercle". The vertical groove extending from the procheilon to the

nasal septum The nasal septum () separates the left and right airways of the Human nose, nasal cavity, dividing the two nostrils. It is Depression (kinesiology), depressed by the depressor septi nasi muscle. Structure The fleshy external end of the nasal s ...

is called the

philtrum The philtrum (, from Ancient Greek , lit. 'love charm') or medial cleft is a vertical indentation in the middle area of the upper lip, common to therian mammals, extending in humans from the nasal septum to the tubercle of the upper lip. Toget ...

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 Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and hea ...

(cells which produce melanin

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

, which give skin its color). Because of this, the

blood vessel Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...

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 A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It ...

of the inside of the mouth. The lip skin is not hairy and does not have

sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin that produce sweat. Sweat glands are a type of exocrine gland, which are glands that produce and secrete substances onto an epithelial s ...

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 The mandibular prominence, or mandibular process is an embryological structure which gives rise to the lower portion of the face. The mandible and lower lip derive from it. The mesenchymal cells within the mandibular prominence condense to form M ...

, a branch of the

first pharyngeal arch The pharyngeal arches, also known as visceral arches'','' are transient structures seen in the embryonic development of humans and other vertebrates, that are recognisable precursors for many structures. In fish, the arches support the gills a ...

. 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

, 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 The lips are a horizontal pair of soft appendages attached to the jaws and are the most visible part of the mouth of many animals, including humans. Mammal lips are soft, movable and serve to facilitate the ingestion of food (e.g. sucklin ...

by the thin lining of the lip itself. Thinning of the vermilion of the upper lip and flattening of the

are two of the facial characteristics of fetal alcohol syndrome, a lifelong

disability Disability is the experience of any condition that makes it more difficult for a person to do certain activities or have equitable access within a given society. Disabilities may be Cognitive disability, cognitive, Developmental disability, d ...

caused by the mother's consumption of alcohol during pregnancy.

Microanatomy

The skin of the lips is

stratified squamous epithelium A stratified squamous epithelium consists of squamous (flattened) epithelial cells arranged in layers upon a basal membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural ...

. The mucous membrane is represented by a large area in the

sensory cortex The sensory cortex can refer sometimes to the primary somatosensory cortex, or it can be used as a term for the primary and secondary cortices of the different senses (two cortices each, on left and right hemisphere): the visual cortex on the occ ...

, and is therefore highly sensitive. The '' frenulum labii inferioris'' is the frenulum of the lower lip. The ''frenulum labii superioris'' is the frenulum of the upper lip.

Nerve supply

Trigeminal nerve In neuroanatomy, the trigeminal nerve (literal translation, lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for Sense, sensation in the face and motor functions ...

** The infraorbital nerve is a branch of the 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 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 Facial expression is the motion and positioning of the muscles beneath the skin of the face. These movements convey the emotional state of an individual to observers and are a form of nonverbal communication. They are a primary means of conveying ...

. All muscles of facial expression are derived from the

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...

of the second

pharyngeal arch The pharyngeal arches, also known as visceral arches'','' are transient structures seen in the Animal embryonic development, embryonic development of humans and other vertebrates, that are recognisable precursors for many structures. In fish, t ...

and are therefore supplied (motor supply) by the nerve of the second pharyngeal arch, the

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of ta ...

(7th

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

). The muscles of facial expression are all specialized members of the

panniculus carnosus The panniculus carnosus is a part of the subcutaneous tissues in vertebrates. It is a layer of striated muscle deep to the panniculus adiposus.McGrath, J.A.; Eady, R.A.; Pope, F.M. (2004). ''Rook's Textbook of Dermatology'' (Seventh Edition). Blac ...

, which attach to the

dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...

and so wrinkle or dimple the overlying skin. Functionally, the muscles of facial expression are arranged in groups around the

orbits In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificia ...

, nose, and mouth. The muscles acting on the lips: * Buccinator * Orbicularis oris (a complex of muscles, formerly thought to be a single

sphincter A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the human bo ...

or ring of muscle) * Anchor point for several muscles ** Modiolus * Lip elevation ** Levator labii superioris ** levator labii superioris alaeque nasi ** Levator anguli oris ** Zygomaticus minor ** Zygomaticus major * Lip depression ** Risorius ** Depressor anguli oris ** Depressor labii inferioris **

Mentalis The mentalis muscle is a paired central Skeletal muscle, muscle of the lower lip, situated at the tip of the chin. It originates from the mentum of the mandible, and inserts into the soft tissue of the chin. It is sometimes referred to as the "pou ...

Functions

Food intake

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, bilabial, and

labiodental In phonetics, labiodentals are consonants articulated with the lower lip and the upper teeth, such as and . In English, labiodentalized /s/, /z/ and /r/ are characteristic of some individuals; these may be written . Labiodental consonants in ...

consonant sounds as well as vowel rounding—and thus are an important part of the speech apparatus. The lips enable

whistling Whistling, without the use of an artificial whistle, is achieved by creating a small opening with one's lips, usually after applying moisture (licking one's lips or placing water upon them) and then blowing or sucking air through the space. Th ...

and the performing of wind instruments such as the trumpet,

clarinet The clarinet is a Single-reed instrument, single-reed musical instrument in the woodwind family, with a nearly cylindrical bore (wind instruments), bore and a flared bell. Clarinets comprise a Family (musical instruments), family of instrume ...

, flute, and saxophone. People who have

hearing loss Hearing loss is a partial or total inability to hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to acquire spo ...

may unconsciously or consciously 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 The McGurk effect is a perceptual phenomenon that demonstrates an interaction between hearing and vision in speech perception. The illusion occurs when the auditory component of one sound is paired with the visual component of another sound, lea ...

Tactile organ

The lip has many nerve endings and reacts as part of the 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

Because of their high number of nerve endings, the lips are an

. 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 Testosterone is the primary male sex hormone and androgen in Male, males. In humans, testosterone plays a key role in the development of Male reproductive system, male reproductive tissues such as testicles and prostate, as well as promoting se ...

on a man's facial structure, the effects of a woman's

oestrogen Estrogen (also spelled oestrogen in British English; see spelling differences) is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three m ...

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. 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 Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

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. 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. The lips may temporarily swell during sexual arousal due to engorgement with blood.

Facial expression

The lips contribute substantially to

s. The lips visibly express emotions such as a

smile A smile is a facial expression formed primarily by flexing the muscles at the sides of the mouth. Some smiles include a contraction of the muscles at the corner of the eyes, an action known as a Duchenne smile. Among humans, a smile expresses d ...

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 and the white roll) is not completely understood. 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 Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

as a

secondary sex characteristic A secondary sex characteristic is a physical characteristic of an organism that is related to or derived from its sex, but not directly part of its reproductive system. In humans, these characteristics typically start to appear during pubert ...

Clinical significance

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 Cyanosis is the change of Tissue (biology), tissue color to a bluish-purple hue, as a result of decrease in the amount of oxygen bound to the hemoglobin in the red blood cells of the capillary bed. Cyanosis is apparent usually in the Tissue (bi ...

; 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. * 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 Herpes simplex, often known simply as herpes, is a viral disease, viral infection caused by the herpes simplex virus. Herpes infections are categorized by the area of the body that is infected. The two major types of herpes are Cold sore, ora ...

) and actinic cheilitis (chronically sun damaged lips). Cleft lip child

Cleft lip A cleft lip contains an opening in the upper lip that may extend into the nose. The opening may be on one side, both sides, or in the middle. A cleft palate occurs when the palate (the roof of the mouth) contains an opening into the nasal cavi ...

is a type of

birth defect A birth defect is an abnormal condition that is present at birth, regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can range from mild to severe. Birth de ...

that can be successfully treated with surgery. *

Carcinoma Carcinoma is a malignancy that develops from epithelial cells. Specifically, a carcinoma is a cancer that begins in a tissue that lines the inner or outer surfaces of the body, and that arises from cells originating in the endodermal, mesoder ...

(a malignant cancer that arises from epithelial cells) at the lips is caused predominantly by using tobacco and overexposure of sunlight. Alcohol appears to increase the carcinoma risk associated with tobacco use. 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.

Society and culture

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. 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 Sigmund Freud ( ; ; born Sigismund Schlomo Freud; 6 May 1856 – 23 September 1939) was an Austrian neurologist and the founder of psychoanalysis, a clinical method for evaluating and treating psychopathology, pathologies seen as originating fro ...

). Lip piercing or

lip augmentation Lip augmentation is a cosmetic procedure that modifies the shape of the lips using fillers, such as collagen or implants. The procedure may be performed to increase lip size, correct asymmetry, create protrusion, or adjust the ratio of the top a ...

is sometimes carried out for cosmetic reasons. Products designed for use on the lips include lipstick,

lip gloss Lip gloss is a cosmetic used primarily to give lips a glossy luster, and sometimes to add a subtle color. It is distributed as a fluid or a soft solid (not to be confused with lip balm, which generally has medical or soothing purposes, or lips ...

and lip balm.

Other animals

In most vertebrates, the lips are relatively unimportant folds of tissue lying just outside the jaws. However, in

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s, they become much more prominent, being separated from the jaws by a deep cleft (a notable exception being the

naked mole-rat The naked mole-rat (''Heterocephalus glaber''), also known as the sand puppy, is a burrowing rodent native to the Horn of Africa and parts of Kenya, notably in Somali regions. It is closely related to the blesmols and is the only species in th ...

, whose lips close behind the front teeth). 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 Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

fish, the lips may be modified to carry sensitive barbels. In birds and turtles, the lips are hard and

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up Scale (anatomy), scales, hair, Nail (anatomy), nails, feathers, horn (anatomy), horns, claws, Hoof, hoove ...

ous, forming a solid

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and ...

. Clevosaurids like '' Clevosaurus'' are notable for the presence of bone "lips"; in these species the tooth-like jaw projections common to all sphenodontians form a beak-like edge around the jaws, protecting the teeth within.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.

References

External links

* *
Anatomy at oralhealth.dent.umich.edu
* * {{Authority control Facial features Mouth Lips Human head and neck Speech organs Human mouth anatomy Digestive system *