onderlip

The onderlip, or lower lip, is the lower portion of the mouth and part of the facial soft tissues. It forms the inferior boundary of the oral fissure and participates in speech, eating, and expression.

Anatomically, the lower lip consists of three layers: the skin externally, the vermilion border in the middle, and the oral mucosa internally. It contains the orbicularis oris muscle, the mentalis muscle near the chin, subcutaneous tissue, and minor labial glands. The labial frenulum connects the inner aspect of the lower lip to the gingiva. The lower lip is bordered at the corners (commissures) and blends with the cheek and chin regions.

Blood supply and innervation are provided by the facial artery via the inferior labial arteries, with venous drainage into the facial vein. Sensory innervation is primarily through the mental nerve, a branch of the mandibular division (V3) of the trigeminal nerve. Motor control of the lips is via the facial nerve (CN VII), which innervates the orbicularis oris and mentalis muscles. Functionally, the lower lip helps seal the mouth, aid in articulation of sounds, and assist in eating, drinking, and facial expression.

Clinical relevance includes susceptibility to trauma and lacerations, which are common in accidents. The lower lip is a frequent site for herpes simplex outbreaks on the vermilion border and for actinic cheilitis due to sun exposure, which can increase the risk of lip cancer with long-term exposure. Cosmetic and reconstructive procedures may address lip shape, volume, or seal integrity after injury or surgery.