Additionally, the lips are involved in mediating sexual attraction and are a well-known erogenous zone exploited during acts of kissing. The large degree of mobility of the lips allows for substantial movements that enable emotional expression such as smiling and frowning. The lips are also required for whistling and the playing of wind instruments. Individuals with hearing loss may rely on the lips to lip-read to understand speech without perceiving sounds. An important component of the speech apparatus, the lips are involved in the creating of bilabial (m, p, b) and labiodental (f, v) consonant sounds and vowel rounding/labialization. The lips play a critical role in breastfeeding, by creating a funnel-shape to allow for suction on the nipple. The lips aid in eating by holding food within the mouth and creating an airtight seal that prevents liquids from spilling out of the oral cavity. The lips surround the oral cavity and play a vital role in mastication, facial expression, phonation, tactile sensation, and intimacy. The transition to the wet labial mucosal is marked by the presence of submucosal salivary glands and the sudden cessation of skin lines. The red line denotes the division between the dry vermilion and the mucous membrane of the oral cavity in both the upper and lower lips. Thus, the blood vessels appear prominent, particularly in those with lighter skin. The characteristic redness of the vermilion zone is secondary to the paucity of melanocytes and high density of superficial vasculature underlying the membrane. The vermilion ranges in color from reddish-pink to brown, depending on ethnicity. Additionally, the vermilion lacks the typical skin appendages seen in the cutaneous lip, bearing no hair follicles or salivary, sweat and sebaceous glands. This membrane is three to five cellular layers in thickness, in contrast to the 16-layer facial skin. The vermilion of the lips is comprised of a modified mucous membrane composed of hairless, highly vascularized, nonkeratinized stratified squamous epithelium. The upper and lower lips connect to the gums by the frenulum labii superioris and frenulum labii inferioris, respectively. Outlining the vermilion borders of the upper and lower lips is a 2 to 3 mm pale convexity known as the white roll, formed by the bulging of the orbicularis oris muscle laying beneath. The inferior margin of the philtrum forms the downward arch of the cupid’s bow, while the underlying fleshy fullness is known as the tubercle or procheilon. The philtrum is believed to serve as a supply of additional skin to be recruited for oral movements requiring stretching of the upper lip. As elasticity is diminished with age, the philtrum takes on a less prominent appearance. The philtral ridges and the philtrum are formed by a unique collection of dermal collagen and dense elastic tissue. The upper lip is characterized by a symmetrical pair of paramedian vertical philtral ridges bordering the central depression known as the philtrum, directly below the nasal septum. The upper and lower lips intersect at the mouth angle, referred to as the commissure. This is the point at which several muscles involved in lip movement attach. While considerations of the lips are often centered on the vermilion zone, the upper lip extends from the nasolabial folds to the inferior margin of the nose, and the lower lip encompasses the region between the lateral commissures and the labiomental crease of the chin. Both the upper and lower lip contains mucosal membrane, vermilion, and cutaneous surfaces. The upper and lower lips are known as, respectively, labium superius oris and labium inferius oris.
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