Topography and Structure
When closed, the lips seal the oral cavity, and their free margins frame the oral fissure (rima oris), the ends of which are called the angles of the mouth (anguli oris). At the angles of the mouth, the lips unite to form the labial commissures (commissurae labiorum).
Blood Supply and Innervation
- Motor innervation of the lips: Facial nerve (n. facialis); the muscles of the lips are supplied by buccal branches (rr. buccales) and the marginal mandibular branch (r. marginalis mandibulae).
- Sensory innervation of the angle of the mouth: Buccal, infraorbital, and mental nerves (nn. buccalis, infraorbitalis, mentalis).
- Blood supply to the lips: Superior and inferior labial arteries (aa. labiales superior et inferior), as well as the mental artery (a. mentalis). Venous drainage occurs via the corresponding vv. labialis superior et labiales inferiores.
Clinical Examination
The condition of the angles of the mouth is evaluated during dental and neurological examinations.
- Dental examination: When examining the vestibule of the oral cavity, the clinician evaluates the color of the vermilion border and the angles of the mouth, as well as the presence of scales and crusts. The examination is performed with jaws closed and lips relaxed.
- Neurological examination: To assess lower facial expression, the patient is asked to show their teeth. Normally, the angles of the mouth are symmetric. The patient is also asked to smile, puff out their cheeks, and blow out a match, during which the lips are protruded forward.
Symptoms of Lesions and Pathology
- Facial nerve palsy: In peripheral nerve lesions, the angle of the mouth on the affected side is drooping, and when showing teeth, the mouth deviates toward the healthy side. Speech and whistling are impaired, and liquids leak from the mouth. Central palsy also presents with a drooping angle of the mouth and deviation of the mouth toward the healthy side upon showing teeth; the muscles of the lower face are affected.
- Tetanus: A characteristic sign is the "sardonic smile" (risus sardonicus). The face becomes rigid, with the angles of the mouth retracted and drooping.
- Eclampsia: During the initial introductory phase of the seizure, lasting up to 30 seconds, the angles of the mouth are pulled downward amidst fine fibrillary twitches of the facial muscles.
- Turner syndrome: Downturned angles of the mouth are among the characteristic facial phenotypic features.