Lips by definition
lip \’lip\ n : : either of the two fleshy folds which surround the opening of the mouth in humans and many other vertebrates and in humans are organs of speech essential to certain articulations; also : the pinkish or reddish margin of a human lip composed of nonglandular mucous membrane and usually exposed when the mouth takes on its natural set.
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.
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, 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 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”. The vertical groove extending from the procheilon to the nasal septum is called the philtrum.
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. 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.
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).
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.