Lip

The lips are a pair of soft, horizontal appendages attached to the jaws.

In many animals, they are the most visible part of the mouth. For mammals, lips are highly specialized tools for survival. They facilitate the ingestion of food through actions like suckling and gulping. They also play a vital role in the articulation of sounds and speech. Beyond these functions, human lips act as a somatosensory organ. This means they are highly sensitive to touch, warmth, and cold.

Structurally, the lips serve as a border between the exterior face and the interior mucous membrane. The skin on the lips is stratified squamous epithelium. This skin is remarkably thin, consisting of only three to five cellular layers. In contrast, typical facial skin can have up to 16 layers.

The lips lack hair and sweat glands. Because they do not produce sweat or body oils, they lack a natural protective layer. This is why lips dry out quickly and become chapped more easily than the rest of the face.

The anatomy of the lips includes several distinct regions. The junction where the lips meet the surrounding facial skin is the vermilion border. The reddish area within these borders is the vermilion zone. On the upper lip, the vermilion border often forms a shape called the Cupid's bow. Directly under the nasal septum, there is a vertical groove called the philtrum, which is Latin for "lower nose." The center of the upper lip also features a fleshy protuberance known as the tubercle, or labial tubercle.

Movement of the lips is controlled by complex muscle groups. These muscles are part of the muscles of facial expression, which are derived from the mesoderm of the second pharyngeal arch.

The orbicularis oris is a complex of muscles that helps shape the mouth. Other muscles, such as the levator labii superioris, work to raise the upper lip. Conversely, the depressor labii inferioris helps lower the bottom lip. These muscles are supplied by the facial nerve, which is the seventh cranial nerve. This coordinated movement allows for a wide range of expressions, from a smile to a frown.

Nerves and blood vessels provide essential sensory and nutritional support. The trigeminal nerve supplies the upper lip via the infraorbital nerve. The mental nerve, a branch of the mandibular nerve, supplies the lower lip and its mucous membrane. Blood is supplied by the facial artery through its superior and inferior labial branches. These branches connect and merge with one another. This rich blood supply is why the lips appear red. In people with light skin, fewer melanocytes allow blood vessels to show through more easily.

The lips serve several critical biological and social functions. For infants, the ability to create a tight seal is essential for breastfeeding. By forming a narrow funnel, the lips increase suction. In communication, lips are necessary for labial and bilabial consonant sounds. They are also used to play wind instruments like the flute or trumpet. Socially, lips are often viewed as symbols of sensuality. Because they are highly sensitive erogenous zones, they play a major role in human intimacy.

Medical conditions can also affect the lips. A blue coloring, known as cyanosis, can occur if blood contains less oxygen. This can be seen in very cold weather or in corpses. Inflammation of the lips is called cheilitis, which includes chapped lips or cold sores. Some people are born with a cleft lip, a birth defect that can be treated with surgery.

Additionally, overexposure to sunlight or tobacco use can lead to carcinoma, a type of cancer. Understanding the lips helps us understand how humans eat, speak, and interact with the world.