7.1. Lips

The lips control the entrance into the mouth. They are positioned by an elaborate musculature and in many animals the lips can manipulate food items extensively before ingestion. The lips have one of the highest concentrations of touch and temperature receptors in the body and they can probe the physical characteristics of potential food items. The development of the lips is modified by sex hormones and they are a target of sexual selection. During kissing behavior, the lips are a central structure of interaction between individuals.

Structure

The upper and lower lips form the border between the mouth and the exterior environment. The juncture where the lips meet the surrounding facial skin is the vermilion border, and the typically reddish area within the borders is called the vermilion zone. The fleshy protuberance located in the center of the upper lip is a tubercle known as procheilon, tuberculum labii superioris, or labial tubercle. The vertical groove extending from the procheilon to the nasal septum is called the philtrum.

The skin of the lip looks pink or red because the its blood vessels are more apparent then in other parts of the skin. This is because:

• The skin of the lips has three to five cellular layers, making it very thin compared to typical facial skin, which has up to 16 layers. 

• It contains fewer melanocytes and less melanin (dark pigment).

• Its keratinocytes contain mostly eleidin instead of keratin, and eleidin is more transparent.

The lip skin is not hairy and does not have sweat glands. Therefore, it does not have the usual protection layer of sweat and body oils which keep the skin smooth, inhibit pathogens, and regulate temperature. The lips therefore dry out faster and become chapped more easily than the surrounding skin.

Internally, both lips have membranous medial attachments to the gums. The frenulum labii inferioris is the frenulum of the lower lip. The frenulum labii superioris is the frenulum of the upper lip.

Innervation

The lips contain one of the highest densities of sensory receptors in our skin, including thermoreceptors, free nerve endings and various types of touch receptors. Neural signals originating at these receptors are transmitted to the brain by branches of the trigeminal nerve (one of the cranial nerves):

• 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.

Musculature

The muscles that move the lips were covered in chapter 7: Ingestion.

Functions

Food intake

Lips are used for holding food or to get it into 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.

In most vertebrates, the lips are relatively small 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, and have muscles embedded in them. In some teleost fishes, the lips may be modified to carry sensitive barbels. In birds and turtles, the lips are hard and keratinous, forming a solid beak.

Communication

Lips are used in vocal sounds, to produce speech and to play musical instruments. People also unconsciously or consciously lip read to understand speech in noisy conditions.

The lips contribute substantially to facial expressions. 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 parabola, respectively. Lips can also be made pouty when whining, or perky to be provocative.

Sensory function

The lip has many nerve endings and is very sensitive to touch and temperature. It provides an important opportunity for problem detection before an item is brought into the mouth. With their high sensory sensitivity, the lips are also an erogenous zone. The lips play a crucial role in kissing and other acts of intimacy.

Mate attraction

Several facial features diverge between men and women after puberty. The sex hormone estrogen causes, among other effects, females to develop thick (full) lips. This is a feature that enhances facial attractiveness as perceived by men. A woman's lipstick (or lip enhancement) attempts to make her more attractive by creating the illusion that she has more estrogen than she actually has and thus indicating higher fertility.

Summary

The lips control the entrance into the mouth, probe the physical characteristics of food items and manipulate them before ingestion. They are sexually dimorphic in humans and their appearance can influence attractiveness.

Key terms

Labia, vermilion, procheilon, nasal septum, labial tubercle, labial frenulum, trigeminal, nerve, lip enhancement, kissing, eleidin

Figure credits

Figure 1 by Mgpapp - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=66033140

Figure 2 by Betta.1 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2687966