Lips in contemporary culture often represent sensuousness, sex, love, and romance. They are also associated with communication, as in the familiar expression “read my lips.”
(labia). (1)In the majority of gnathostomatous vertebrates, the immobile cutaneous folds that surround the mouth. Lips are absent in tortoises, birds. and adult cloacal mammals. owing to the development of a horny beak on the jaws. A distinction is made between the upper and lower lips. In fish the lips are usually well supplied with gustatory and tactile organs and aid in catching prey. In the majority of amphibians the lips serve to seal the oral cavity during respiration. In reptiles (snakes and lizards) the lips are distinctly expressed and covered externally with horny labial plates. In mammals the lips are mobile, which acquires special significance in adaptation of the young to sucking and of the adult animals to active food capture. Striated musculature develops in the lips. In the elephant, swine, and tapir the upper lip is greatly extended and forms the lower side of the trunk or snout. In cyclostomes the so-called upper lip serves as a sucker. Certain parts of the mouth apparatus are called labia in a number of invertebrates.
In man. the lips are mobile folds that circumscribe the oral cavity anteriorly and are formed of skin and mucous membrane, between which are enclosed the orbicularis oris muscle and the small mimetic muscles. The site of transition from skin to mucous membrane is the vermilion border, which abounds in blood vessels. Sensory innervation of the lips is from the trigeminal nerve.
In anthropology lips are classified according to the thickness, direction, and contour of the upper lip and the width of the mouth opening. Lips are divided, according to thickness, into thin, average, thick, and inflated. The upper lip may protrude (procheilia), have a vertical profile (orthocheilia), or, more rarely, recede (opisthocheilia). The thickest (inflated) lips and procheilia are characteristic of equatorial (Negroid and Australoid) races. Orthocheilia is characteristic of Europoids. The thinnest lips are found in certain peoples of northern Europe and Asia. The upper lip may vary also in contour—concave, straight, or convex. This last is especially characteristic of the pygmies of central Africa and the Semangs (Malacca peninsula). The height and profile of the upper lip, the thickness of the lips, and the width of the mouth vary also with age and sex. Thickness and procheilia diminish with age and sex. Thickness and procheilia diminish with age (after 25 years), while the height of the upper lip and the width of the mouth increase.