Structure/Morphology
The lips are the soft, visible part of the humans’ mouth and comprise the upper and the lower lips. Together these serve as the gatekeepers of the oral cavity.
Embryologically, the lower lip is derived from the first pharyngeal arch and covers the body of the mandible anteriorly. It’s lowered by the facial muscles, such as depressor labii inferioris, depressor anguli inferioris, and mentalis, which help to open the oral orifice. The orbicularis oris helps in closing the mouth.
The upper lip covers the body of the maxilla anteriorly. It is raised by facial muscles, such as levator labii superioris, levator labii superioris alaeque nasi, levator anguli oris, and zygomaticus major and minor, which help open the oral orifice. The center of the upper lip has a labial tubercle. The tubercle is situated at the base of the philtrum, a vertical groove extending from the labial tubercle to the nasal septum.
The lips are richly innervated structures. The upper lip receives its cutaneous sensory innervation from the infraorbital nerve (branch of maxillary nerve), which also innervates the facial skin between the upper lip and the lower eyelid. The lower lip receives its cutaneous sensory innervation from the mental nerve (branch of inferior alveolar nerve).
Arterial supply to the lips comes from the superior and inferior labial branches of the facial artery (a branch of the external carotid artery).
Key Features/Anatomical Relations
The surface of the lips is covered by a thin skin comprising of stratified squamous epithelium. It is non-hairy and lacks sweat glands.
Externally, the lips’ skin forms a junction with the surrounding facial skin, which is marked by the vermilion border (also called the cupid’s bow in the case of the upper lip). Internally, it forms a junction with the mucous membrane inside the oral cavity.
Function
Serving as the gatekeepers of the oral cavity, the lips help in food intake and in articulation of speech. Moreover, being tactile sensory organs and due to their rich sensory innervation, they also serve as erogenous zones in acts of intimacy. Additionally, lips also contribute to the facial expressions.
List of Clinical Correlates
- Cleft lip (and palate)
- Cheilitis
