Lips

The lips (labia oris) are mobile, fibromuscular folds that establish the anterior boundary of the oral cavity and surround the rima (oral fissure).

Composition

Moving from the external to the internal surface, the lips are structurally composed of:

• External layer: Skin

• Intermediate layer: The Orbicularis oris muscle, the labial vessels, nerves, areolar tissue, adipose tissue (fat), and numerous small labial salivary glands.

• Internal layer: Mucous membrane

In the midline, the internal mucous membrane of each lip reflects onto the corresponding gum, forming a structural fold of mucous membrane known as the frenulum. Notably, the upper labial frenulum is typically larger than the lower.

Functions

The lips are important in speech, facial expression, mastication, and maintaining oral competence.

Innervation

The upper lip receives sensory innervation from the infraorbital nerve (CN V2), while the lower lip is supplied by the mental nerve (CN V3). Motor supply to the lip muscles is provided by the facial nerve (CN VII).

Vasculature

Arterial supply is mainly through the superior and inferior labial branches of the facial artery.

Lymphatics

Lymphatic drainage of the upper lip is mainly to the submandibular lymph nodes, whereas the central lower lip drains to the submental nodes.