Lip

# Overview
Lips are a visible organ at the mouth of humans and many animals.  Both lips are soft, protruding, movable, and serve primarily for food intake, as a tactile sensory organ, and in articulation of speech.

# Anatomical basics of the human lip
One differentiates between the Upper (Labium superioris) and lower lip (Labium inferioris). The lower lip is usually somewhat larger. The border between the lips and the surrounding skin is referred to as the vermilion border, or simply the vermilion. The vertical groove on the upper lip is known as the philtrum. The entire skin between the upper lip and the nose is referred to as the "ergotrid".
The skin of the lip, with three to five cellular layers, is very thin compared to typical face skin, having up to 16 layers. With light skin color, the lip skin contains no melanocyte (pigment cells, 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 thicker.
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, kill 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.
- 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 specialised members of the paniculus 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
-rbicularis oris
- buccinator
- orbicularis oris
- anchor point for several muscles
modiolus
- modiolus
- lip elevation
levator labii superioris
levator labii superioris alaeque nasi
levator anguli oris
zygomaticus minor
zygomaticus major
- 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
- 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 very 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, and 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.

## 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 aide for exploring unknown objects for babies and toddlers.

## 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 like the trumpet, clarinet, and flute.

## Facial expressions
See Full Article: facial expression.
The lips visibly express emotions.

## Erogenous zone
Because of its high amounts of nerve endings, the lips make an erogenous zone. Lips play a crucial role in kissing and other acts of intimacy.
Lips are a visible expression of fertility. It has been shown that the more estrogen a woman has the fuller her lips and that full lips are considered attractive.(Indeed lipstick "tricks" men into thinking that a women has more estrogen than she actually has and thus finding her more attractive.

# Symbolic meaning
Lips are often viewed as a symbol for sensuality. This has many origins; above all that they are very sensitive as a tactile organ and feel pleasantly soft. It has been suggested that female lips are seen as sexually attractive because they mimic the appearance and sexual swelling of the labia of the vulva, and that the lips are therefore a secondary sex characteristic.     Additionally, they are a part of the mouth and so are associated with its symbolic connections (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 always 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 or misallignment 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.
- Carcinoma 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 of tobacco use.
- Infections from lip rings.