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Lips
The lips are mobile musculofibrous folds surrounding the oral opening and forming the anterior boundary of the oral cavity. They contain the orbicularis oris muscle and are covered externally by skin, internally by oral mucosa, and between these surfaces by the characteristic vermilion zone.
RegionHead and Neck
SystemDigestive System
The lips are two mobile musculofibrous folds that surround the entrance to the oral cavity. The upper lip and lower lip meet laterally at the angles of the mouth and enclose the oral fissure. Together, they form the anterior boundary of the oral vestibule and play important roles in feeding, mastication, swallowing, speech, facial expression, and oral continence.
The structural core of each lip is formed primarily by the orbicularis oris muscle, together with fibers from several other muscles of facial expression. Externally, the lips are covered by skin, while internally they are lined by oral mucosa. Between these surfaces is the highly vascular vermilion zone, which produces the characteristic reddish appearance of the lips.
The lips contain a rich vascular and sensory supply. The superior and inferior labial arteries are major arterial contributors, while motor innervation of the lip musculature is provided by the facial nerve. General sensation is supplied predominantly by branches of the trigeminal nerve.
Location of the Lips
The lips occupy the anterior opening of the oral cavity and extend between the surrounding facial regions and the oral vestibule.
The upper lip lies inferior to the nose, while the lower lip lies superior to the chin.
Upper and Lower Lips
The lips are divided into:
- Upper lip, or labium superius
- Lower lip, or labium inferius
They meet on either side at the angles of the mouth.
Oral Fissure
The oral fissure is the opening between the upper and lower lips.
Its shape and width change continuously with movements of the lips, mandible, and associated facial muscles.
Angles of the Mouth
The upper and lower lips meet laterally at the angles of the mouth, also called the oral commissures.
These regions contain complex interlacing fibers from multiple muscles of facial expression.
Oral Commissures
The oral commissures form the lateral endpoints of the oral fissure.
Movements of the commissures are particularly important in facial expression and are produced by coordinated contraction of several facial muscles.
General Structure of the Lips
Each lip contains several tissue layers arranged between its external and internal surfaces.
These include skin, subcutaneous connective tissue, muscle, submucosal tissue containing labial glands, and oral mucosa.
Layers of the Lips
| Layer | Main Features |
|---|---|
| Skin | External covering containing typical cutaneous structures |
| Subcutaneous tissue | Contains connective tissue, vessels and nerves |
| Muscular layer | Dominated by orbicularis oris and interlacing facial muscle fibers |
| Submucosa | Contains labial salivary glands, vessels and nerves |
| Oral mucosa | Lines the internal surface of the lip |
External Surface
The external portion of the lip is covered by skin continuous with the surrounding facial skin.
This region contains hair follicles, sebaceous glands, sweat glands, sensory nerve endings, and other typical components of skin.
Internal Surface
The internal surface of each lip faces the oral vestibule and is lined by labial mucosa.
This surface is moist and continuous with the mucosa of the cheeks and gingivolabial regions.
Vermilion Zone
The vermilion zone is the transitional region between the external skin and internal oral mucosa.
Its characteristic reddish color results largely from the thin epithelial covering and the visibility of the underlying vascular network.
Vermilion Border
The vermilion border marks the transition between the facial skin and the vermilion portion of the lip.
It is an important external landmark and is especially distinct on the upper lip.
Histology of the Vermilion
The vermilion region has a relatively thin epithelial covering over highly vascular connective tissue.
Capillary loops lie relatively close to the surface, contributing to the red coloration visible through the epithelium.
Upper Lip
The upper lip extends from the oral fissure inferiorly toward the base of the nose superiorly.
Its external surface contains several characteristic landmarks, including the philtrum and Cupid's bow.
Philtrum
The philtrum is the vertical midline groove on the external surface of the upper lip extending between the base of the nose and the vermilion border.
It is bordered by paired vertical ridges known as the philtral columns.
Philtral Columns
The philtral columns form the raised lateral boundaries of the philtrum.
They contribute to the characteristic surface contour of the central upper lip.
Cupid's Bow
The Cupid's bow is the curved contour of the central vermilion border of the upper lip.
Its central depression and paired peaks correspond closely with the inferior ends of the philtral columns.
Upper Lip Tubercle
The central portion of the upper lip may project slightly anteriorly to form the labial tubercle.
This contributes to the normal three-dimensional contour of the upper lip.
Lower Lip
The lower lip extends from the oral fissure toward the mentolabial region.
Its contour is influenced by orbicularis oris, surrounding facial muscles, connective tissue, and the underlying mandible.
Mentolabial Sulcus
The mentolabial sulcus is the transverse depression separating the lower lip from the prominence of the chin.
Its depth and shape are influenced by the underlying skeletal and muscular anatomy.
Orbicularis Oris
The orbicularis oris forms the principal muscular component of the lips.
Rather than functioning as a simple circular sphincter, it consists of intrinsic fibers and fibers contributed by surrounding muscles that interlace around the oral aperture.
Organization of Orbicularis Oris
Orbicularis oris contains fibers with complex origins and insertions around the mouth.
Some fibers are intrinsic to the lips, while others enter from muscles that converge on the oral commissures.
Function of Orbicularis Oris
Orbicularis oris closes the oral fissure and controls the shape and position of the lips.
Its actions include compression of the lips against the teeth, pursing, protrusion, and fine adjustments required for speech and feeding.
Modiolus
The modiolus is a dense fibromuscular region located near each angle of the mouth.
Fibers from orbicularis oris and several other muscles of facial expression converge and interlace within this region.
Muscles Associated With the Lips
Several muscles act on the lips and oral commissures:
- Orbicularis oris
- Buccinator
- Levator labii superioris
- Levator labii superioris alaeque nasi
- Zygomaticus major
- Zygomaticus minor
- Levator anguli oris
- Risorius
- Depressor anguli oris
- Depressor labii inferioris
- Mentalis
Levator Labii Superioris
The levator labii superioris elevates the upper lip and contributes to movements associated with facial expression.
Levator Labii Superioris Alaeque Nasi
The levator labii superioris alaeque nasi elevates the upper lip and can also affect the ala of the nose.
Zygomaticus Major
The zygomaticus major draws the angle of the mouth superiorly and laterally.
It is an important muscle in smiling.
Zygomaticus Minor
The zygomaticus minor contributes to elevation of the upper lip.
Levator Anguli Oris
The levator anguli oris elevates the angle of the mouth.
Its fibers participate in the muscular complex surrounding the oral commissure.
Risorius
The risorius retracts the angle of the mouth laterally.
Its size and development vary considerably between individuals.
Depressor Anguli Oris
The depressor anguli oris draws the angle of the mouth inferiorly.
Depressor Labii Inferioris
The depressor labii inferioris draws the lower lip inferiorly and slightly laterally.
Mentalis
The mentalis elevates and protrudes the lower lip while wrinkling the skin of the chin.
It contributes to control of the central lower lip.
Buccinator and the Lips
Buccinator fibers converge toward the angles of the mouth and blend with the orbicularis oris complex.
This anatomical continuity helps coordinate movements of the lips and cheeks during mastication, swallowing, speech, and facial expression.
Labial Mucosa
The internal surfaces of the lips are covered by labial mucosa.
This mucosa is generally lined by nonkeratinized stratified squamous epithelium suited to the flexible environment of the oral vestibule.
Labial Submucosa
Deep to the labial mucosa is a submucosal layer containing connective tissue, blood vessels, nerves, and numerous minor salivary glands.
Labial Glands
The labial glands are minor salivary glands located predominantly on the internal aspect of the lips.
Their small ducts open directly onto the labial mucosa.
Function of the Labial Glands
Secretions from the labial glands contribute to lubrication and moisture of the oral mucosal surface.
They form part of the widely distributed minor salivary gland system of the oral cavity.
Labial Frenula
The upper and lower lips are connected to the adjacent gingival region by folds of mucosa known as the labial frenula.
These folds are most prominent in the midline.
Superior Labial Frenulum
The superior labial frenulum connects the internal midline of the upper lip with the maxillary gingival region.
Inferior Labial Frenulum
The inferior labial frenulum connects the internal midline of the lower lip with the mandibular gingival region.
Gingivolabial Sulcus
The reflection of mucosa between the lip and gingiva forms the gingivolabial sulcus.
This space forms part of the anterior oral vestibule.
Blood Supply of the Lips
The lips have a rich arterial blood supply derived predominantly from branches of the facial artery.
The most important direct vessels are the superior and inferior labial arteries.
Superior Labial Artery
The superior labial artery usually arises from the facial artery near the angle of the mouth.
It passes medially within the upper lip and anastomoses with its counterpart from the opposite side.
Inferior Labial Artery
The inferior labial artery is also usually a branch of the facial artery.
It travels medially within the lower lip and communicates with the corresponding artery from the opposite side.
Arterial Anastomoses
The labial arteries form extensive anastomoses within the lips.
This rich vascular network contributes to the ability of lip wounds to bleed substantially but also supports tissue perfusion and healing.
Additional Arterial Contributions
The upper lip may receive additional arterial contributions from vessels associated with the infraorbital and nasal regions, while the lower lip may receive contributions from branches associated with the mental region.
Arterial Supply Summary
| Region | Major Arterial Supply |
|---|---|
| Upper lip | Superior labial artery, with additional regional contributions |
| Lower lip | Inferior labial artery, with additional regional contributions |
| Main parent vessel | Facial artery |
Venous Drainage
Venous blood from the lips drains through a network of veins that communicates extensively with facial venous channels.
Much of this blood ultimately enters the facial vein.
Facial Vein
The facial vein is a major venous drainage pathway of the superficial face.
It receives tributaries from regions around the mouth and continues inferiorly toward larger veins of the neck.
Venous Communications
Facial veins communicate with other superficial and deep venous pathways.
These communications are clinically relevant because facial infections may potentially spread along interconnected venous channels.
Motor Innervation
The muscles controlling the lips are muscles of facial expression and receive motor innervation from the facial nerve (cranial nerve VII).
Different facial nerve branches supply muscles acting on the upper and lower lips.
Buccal Branches of the Facial Nerve
Buccal branches of the facial nerve provide motor fibers to muscles around the upper lip and oral region, including portions of the orbicularis oris complex.
Marginal Mandibular Branch
The marginal mandibular branch of the facial nerve supplies several muscles acting on the lower lip.
Its course along the lower face makes it particularly important during surgical procedures near the inferior border of the mandible.
Sensory Innervation
General sensation from the lips is supplied by branches of the trigeminal nerve (cranial nerve V).
The upper and lower lips receive sensory fibers from different trigeminal divisions.
Upper Lip Sensation
The upper lip receives general sensory innervation primarily through the infraorbital nerve, a continuation of the maxillary division of the trigeminal nerve (CN V2).
Inferior Palpebral, External Nasal and Superior Labial Branches
After emerging through the infraorbital foramen, the infraorbital nerve divides into terminal branches.
Its superior labial branches provide sensory fibers to the upper lip.
Lower Lip Sensation
The lower lip receives general sensory innervation primarily through the mental nerve.
The mental nerve is a terminal branch of the inferior alveolar nerve from the mandibular division of the trigeminal nerve (CN V3).
Mental Foramen
The mental nerve emerges onto the face through the mental foramen of the mandible.
It then divides into branches supplying the lower lip, chin, and adjacent tissues.
Sensory Innervation Summary
| Region | Primary Sensory Nerve | Trigeminal Division |
|---|---|---|
| Upper lip | Infraorbital nerve | Maxillary nerve, V2 |
| Lower lip | Mental nerve | Mandibular nerve, V3 |
Motor Versus Sensory Innervation
| Function | Nerve |
|---|---|
| Movement of lips | Facial nerve, CN VII |
| Upper lip sensation | Infraorbital nerve, CN V2 |
| Lower lip sensation | Mental nerve, CN V3 |
Lymphatic Drainage
The lymphatic drainage of the lips differs between the upper and lower lips and between the central and lateral portions of the lower lip.
This pattern is clinically important in infections and malignant lesions.
Upper Lip Lymphatic Drainage
Lymph from the upper lip drains predominantly toward the submandibular lymph nodes.
Lateral Lower Lip Drainage
The lateral portions of the lower lip also drain primarily toward the submandibular lymph nodes.
Central Lower Lip Drainage
The central portion of the lower lip commonly drains initially toward the submental lymph nodes.
Deep Cervical Nodes
Lymph from the submental and submandibular nodes ultimately reaches the deep cervical lymph nodes.
Lymphatic Drainage Summary
| Lip Region | Principal Initial Nodes |
|---|---|
| Upper lip | Submandibular nodes |
| Lateral lower lip | Submandibular nodes |
| Central lower lip | Submental nodes |
Functions of the Lips
The lips perform several interconnected functions related to the digestive system, communication, and facial expression.
- Control entry to the oral cavity
- Assist with ingestion of food and liquids
- Maintain oral continence
- Participate in sucking
- Assist mastication
- Contribute to swallowing
- Participate in speech articulation
- Produce facial expressions
- Provide tactile sensory information
Lips During Feeding
The lips help capture and retain food and liquids within the oral cavity.
Closure of the oral fissure prevents anterior escape of oral contents during mastication and swallowing.
Lips During Sucking
The lips form a seal around the nipple or another object during sucking.
Orbicularis oris and associated muscles help maintain this seal while pressure changes are generated within the oral cavity.
Lips During Mastication
During chewing, the lips cooperate with the cheeks and tongue to retain food within the oral cavity.
The orbicularis oris controls the anterior boundary of the oral vestibule while the buccinator controls its lateral boundary.
Lips During Swallowing
Lip closure contributes to the oral phase of swallowing by maintaining an anterior seal while the tongue manipulates and propels the bolus posteriorly.
Lips and Speech
The lips are essential articulatory structures.
Precise changes in lip closure, protrusion, retraction, and shape contribute to the production of numerous speech sounds.
Bilabial Sounds
Bilabial speech sounds are produced by bringing the upper and lower lips together.
They depend on coordinated activity of orbicularis oris and associated facial muscles.
Labiodental Sounds
Labiodental sounds involve contact or approximation between the lower lip and upper teeth.
The ability to position the lower lip precisely depends on intact muscular and sensory control.
Lips and Facial Expression
The lips and oral commissures are moved by a complex network of muscles of facial expression.
Elevation, depression, retraction, protrusion, compression, and asymmetrical movements contribute substantially to visible facial expressions.
Development of the Upper Lip
The upper lip develops through fusion of facial prominences during embryonic development.
The central portion is associated with tissues derived from the merged medial nasal prominences, while lateral portions are associated with the maxillary prominences.
Development of the Lower Lip
The lower lip develops principally from tissues associated with the paired mandibular prominences of the first pharyngeal arch.
These prominences merge in the midline early in facial development.
Intermaxillary Segment
The merged medial nasal prominences contribute to the intermaxillary segment.
This embryological region contributes to the central upper lip, including the philtral region.
Cleft Lip
Cleft lip results from abnormal fusion of facial processes involved in formation of the upper lip.
It may occur on one or both sides and can vary substantially in extent.
Unilateral Cleft Lip
A unilateral cleft affects one side of the upper lip and reflects failure of normal fusion between developing facial tissues on that side.
Bilateral Cleft Lip
A bilateral cleft involves both sides of the upper lip and may isolate the central upper lip segment to varying degrees.
Cleft Lip and Palate
Cleft lip may occur alone or together with cleft palate.
Although these abnormalities are developmentally related, formation of the lip and palate involves distinct developmental events.
Facial Nerve Palsy
Weakness or paralysis of the facial nerve can impair movements of the lips.
Patients may have difficulty maintaining oral closure, retaining food and liquids, articulating speech, and producing symmetrical facial expressions.
Lower Lip Weakness
Injury to the marginal mandibular branch of the facial nerve can weaken muscles controlling the lower lip.
This may produce visible asymmetry, particularly during movement.
Infraorbital Nerve Injury
Damage to the infraorbital nerve can reduce sensation in the upper lip together with other areas of its cutaneous distribution.
Mental Nerve Injury
Damage to the mental nerve can produce altered or reduced sensation involving the lower lip and chin.
This nerve is particularly relevant during dental and mandibular procedures.
Local Anesthesia
Knowledge of trigeminal sensory innervation allows regional anesthesia of the lips through targeted nerve blocks.
Infraorbital nerve anesthesia can affect sensation of the upper lip, while mental nerve anesthesia can affect the lower lip.
Lip Lacerations
Lacerations of the lips may bleed substantially because of their rich vascular supply.
Injuries crossing the vermilion border require careful anatomical alignment because even small irregularities can alter the visible contour of the lip.
Labial Artery Injury
Deep lip injuries can damage the superior or inferior labial arteries.
The extensive arterial anastomoses within the lips contribute to both bleeding and robust tissue perfusion.
Mucocele
A mucocele is a mucus-containing lesion commonly associated with disruption of a minor salivary gland duct.
The lower lip is a frequent location because minor labial glands can be injured by biting or other trauma.
Angular Cheilitis
Inflammatory changes can occur at the oral commissures, where the upper and lower lips meet.
The anatomy of this region can permit moisture and mechanical stress to concentrate within the commissural folds.
Cheilitis
Cheilitis is a general term for inflammation involving the lips.
Different forms may affect the vermilion, commissures, mucosal surface, or surrounding skin.
Lip Carcinoma
Malignant tumors may arise from tissues of the lips, particularly the epithelial surfaces.
The lower lip is an important site of squamous cell carcinoma of the oral and perioral region.
Lymphatic Spread of Lip Lesions
The location of a lip lesion influences its likely initial lymphatic drainage.
Central lower lip lesions may spread toward submental nodes, while upper and lateral lower lip regions commonly drain toward submandibular nodes.
Clinical Examination of the Lips
Examination includes inspection of the external skin, vermilion, oral commissures, and internal labial mucosa.
The lips can also be palpated to evaluate submucosal structures, masses, tenderness, and abnormalities of the minor salivary glands.
Surface Landmarks
Important external landmarks include:
- Philtrum
- Philtral columns
- Cupid's bow
- Vermilion border
- Upper lip tubercle
- Oral commissures
- Mentolabial sulcus
Internal Landmarks
Important structures visible on the internal surfaces of the lips include the labial mucosa, superior and inferior labial frenula, and the mucosal reflections forming the oral vestibule.
Lips and the Oral Vestibule
The lips form the anterior wall of the oral vestibule.
The vestibule lies between the lips and cheeks externally and the teeth and gingivae internally.
Lips and the Teeth
The lips normally rest against the anterior surfaces of the teeth and gingivae.
Their position is influenced by dental alignment, skeletal relationships, muscular tone, and soft tissue anatomy.
Lips and the Maxilla
The contour and position of the upper lip are influenced by the maxillary alveolar process and upper teeth.
Changes in maxillary projection or dental position can therefore alter the external profile of the upper lip.
Lips and the Mandible
The lower lip is related to the mandibular alveolar process and lower teeth.
Its position and contour are influenced by mandibular projection, dental relationships, and the activity of surrounding muscles.
Key Differences Between Upper and Lower Lips
| Feature | Upper Lip | Lower Lip |
|---|---|---|
| Major surface landmark | Philtrum and Cupid's bow | Related inferiorly to mentolabial sulcus |
| Main sensory nerve | Infraorbital nerve | Mental nerve |
| Trigeminal division | V2 | V3 |
| Main labial artery | Superior labial artery | Inferior labial artery |
| Principal lymphatic drainage | Submandibular nodes | Submental centrally, submandibular laterally |
Key Features of the Lips
| Feature | Key Point |
|---|---|
| Principal muscle | Orbicularis oris |
| External covering | Skin |
| Internal covering | Oral mucosa |
| Transition region | Vermilion zone |
| Main arterial supply | Superior and inferior labial branches of facial artery |
| Motor innervation | Facial nerve, CN VII |
| Upper lip sensation | Infraorbital nerve, CN V2 |
| Lower lip sensation | Mental nerve, CN V3 |
| Minor salivary glands | Labial glands |
| Major functional roles | Feeding, oral closure, speech and facial expression |
Anatomical and Functional Importance
The lips form the mobile anterior boundary of the oral cavity and combine skin, mucosa, skeletal muscle, connective tissue, glands, vessels, and nerves within a relatively compact structure. Their principal muscular framework is the orbicularis oris, which integrates with numerous surrounding muscles at the oral commissures.
The upper and lower lips have distinct sensory innervation. The upper lip is supplied primarily by the infraorbital nerve from the maxillary division of the trigeminal nerve, while the lower lip is supplied primarily by the mental nerve from the mandibular division. Motor control of both lips is provided by branches of the facial nerve.
The lips are essential for maintaining closure of the oral cavity, controlling food and liquids during feeding, sucking, mastication, swallowing, speech articulation, and facial expression. Their rich arterial supply, characteristic lymphatic drainage, and close relationship to facial nerves and oral structures also make detailed lip anatomy important in dentistry, facial trauma, reconstructive surgery, neurological examination, and evaluation of oral disease.
Published on October 1, 2026
Last updated on October 1, 2026
Last updated on October 1, 2026