Lip

The lip is one of the two fleshy folds that frame the mouth in humans and many animals. It consists of an outer skin, an inner mucous membrane, and an underlying core of muscle, connective tissue, and minor glands. The lips are divided into the upper lip and the lower lip, separated at the midline by the philtrum, a vertical groove that continues from the nose to the vermilion border. The visible colored portion of the lip is the vermilion, which has a relatively thin epithelium and richly vascularized tissue beneath, giving the lip its characteristic red or pink appearance.

Key structures include the orbicularis oris muscle, which encircles the mouth and powers lip movements; the vermilion border, labial mucosa on the inner surface, and the submucosa containing minor salivary glands. The lips play an essential role in articulation, feeding, and facial expression, helping to seal the mouth during swallowing and enabling precise lip movements required for phonemes, swallowing, and sucking in infants.

Nerve and blood supply are well developed. Motor innervation is provided by the facial nerve (cranial nerve VII), which drives the muscles of the lips. Sensory input mainly comes from branches of the trigeminal nerve (cranial nerve V) to the upper and lower lips. Blood is supplied by the superior and inferior labial arteries, branches of the facial artery, with venous drainage largely through the facial vein.

Common clinical concerns include cheilitis (inflammation of the lips), angular cheilitis at the corners of the mouth, and herpes labialis (cold sores). Lip health reflects exposure, hydration, and overall oral hygiene.