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 an erogenous zone when used in kissing and other acts of intimacy.
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.
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.
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.
The lips serve for creating different sounds—mainly labial, bilabial, and labiodental consonant sounds as well as vowel rounding—and thus are 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. People who have hearing loss may unconsciously or consciously lip read to understand speech without needing to perceive the actual sounds.
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.
A womans lips are also a visible expression of her fertility. In studies performed on the science of human attraction, psychologists have concluded that a womans 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 mans facial structure, the effects of a womans 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, characteristics which are perceived as more feminine. Surveys performed by sexual psychologists[who?] have also found that universally, men find a womans full lips to be more sexually attractive than lips that are less so.[citation needed] A womans lips are therefore sexually attractive to males because they serve as a biological indicator of a womans health and fertility. A womans lipstick (or collagen 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.
Facial expression - 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.