lip

Lip
lips
Latin	labia oris
Artery	inferior labial, superior labial
Vein	inferior labial, superior labial
Nerve	frontal, infraorbital

Lips are a visible body part at the mouth of humans and many animals. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech. Human lips are a tactile sensory organ, and can be erogenous when used in kissing and other acts of intimacy.

Anatomical basics of the human lip

The upper and lower lips are referred to as the "Labium superius oris" and "Labium inferius oris", respectively.^[1][2] The juncture where the lips meet the surrounding skin of the mouth area is the vermilion border^[3], and the typically reddish area within the borders is called the vermilion zone.^[4] The vermilion border of the upper lip is known as the cupid's bow.^[5] The fleshy protuberance located in the center of the upper lip is a tubercle known by various terms including the procheilon (also spelled prochilon), the "tuberculum labii superioris", and the "labial tubercle".^[6] The vertical groove extending from the procheilon to the nasal septum is called the philtrum.^[7]

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 vessels 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 glands or sebaceous glands. 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.

Anatomy in detail

The skin of the lips is stratified squamous epithelium. The mucous membrane is represented by a large area in the sensory cortex, 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.

Sensory nerve supply

• Trigeminal nerve
  • The infraorbital nerve is a branch of the maxillary branch. It supplies not only the upper lip, but 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. It supplies the lips by its superior and inferior labial branches, each of which bifurcate and anastomose with their companion artery from the other side.

Muscles acting on the lips

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 orbits, nose and mouth.

The muscles acting on the lips:

• Sphincters of the oral orifice
  • Buccinator
  • Orbicularis oris
• 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

Functions of the lips

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 the labial, bilabial, and labiodental consonant sounds - and thus create 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.

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 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. 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.^[8] A woman's lips are therefore sexually attractive to males because they serve as a biological indicator of a woman's health and fertility. As such, a woman's lipstick (or collagen lip enhancement) takes advantage of this fact by "tricking" men into thinking that a woman has more oestrogen than she actually has, and thus that she is more fertile and attractive.^[9]

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.^[10]

Facial expressions

This section requires expansion.

The lips visibly express emotions such as a smile or frown. Lips can also be made pouty when whining, or perky to be provocative.

Symbolic meaning

Lips are often viewed as a symbol for 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.^[11]

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).

Changes to the lip

• 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.
• Lips can (temporarily) swell. The reasons for this are varied and can be from sexual stimulation, injuries and side effects of medications, sleeping, or misalignment of teeth.
• Cracks or splits in the angles of the lips could be the result of an inflammation of the lips, Angular cheilitis.

Diseases

As an organ of the body, the lip can be a focus of disease or show symptoms of a disease:

• Lip herpes (technically Herpes labialis, a form of herpes simplex) is a viral infection which appears in the formation of painful blisters at the lip. It is also commonly known as a cold sore.
• Carcinoma (a malignant cancer that arises from epithelial cells) at the lips, is caused predominantly by using tobacco and overexposure of sunlight. To a lesser extent, it could also come from lack of oral hygiene or poor fitting dentures. Alcohol appears to increase the carcinoma risk associated with tobacco use.

In other animals

In most vertebrates, the lips are relatively unimportant folds of tissue lying just outside the jaws. However, in mammals, they become much more prominent, being separated from the jaws by a deep cleft. 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 barbels. In birds and turtles, the lips are hard and keratinous, forming a solid beak.^[12]

Additional images

• 

  The labial coronary arteries, etc.

• 

  Chapped lips

• 

  Cleft lip

• 

  The Kiss, by Francesco Hayez, 1859

• 

  Lip balm

• 

  Makonde woman in full dress

• 

  Lipstick

• 

  Pierced lips

See also

• Chapped lips
• Cleft lip
• Cupid's bow
• Facial expression
• Herpes
• Kiss
• Lip balm
• Lip enhancement
• Lip gloss
• Lip piercing
• Lipstick

References

Further reading

• Tomiyama N, Ichida T, Yamaguchi K (2004). "Electromyographic activity of lower lip muscles when chewing with the lips in contact and apart" (abstract). Angle Orthod 74 (1): 31–6. PMID 15038488. http://journals.allenpress.com/jrnlserv/?request=get-abstract&issn=0003-3219&volume=74&page=31. 
• Bisson M, Grobbelaar A (2004). "The esthetic properties of lips: a comparison of models and nonmodels" (abstract). Angle Orthod 74 (2): 162–6. PMID 15132441. http://journals.allenpress.com/jrnlserv/?request=get-abstract&issn=0003-3219&volume=74&page=162. 
• McMinn, R. M. H.; Last, R. J. (1994). Last's anatomy, regional and applied. Edinburgh: Churchill Livingstone. ISBN 0-443-04662-X. 

External links

Wikiquote has a collection of quotations related to: Lips

Wikimedia Commons has media related to: Lips

Look up lip in Wiktionary, the free dictionary.

• ent/7 at eMedicine
• hednk-030 — Embryology at UNC
• Anatomy at oralhealth.dent.umich.edu