The lips are central to feeding and communication. While people are familiar with the upper and lower lips, the specific point where they meet on either side often lacks a common name in everyday conversation. This junction is a specialized piece of anatomy, playing a key role in movement, expression, and health. Understanding this area requires appreciating its distinct structure and function.
The Anatomical Terminology
The corners of the lips have a precise anatomical designation: the oral commissures, or sometimes the labial commissures. A commissure is the lateral point where the superior labium (upper lip) meets the inferior labium (lower lip). This junction marks the end of the vermillion border, the distinct line separating the reddish lip tissue from the surrounding facial skin. The commissure is a soft-tissue hinge that allows the mouth to open wide and serves as a common reference point for clinicians assessing facial symmetry and oral health.
Muscles Governing Movement
The movement of the oral commissure is intricate, controlled by a convergence of numerous facial muscles at a hub called the modiolus. The primary muscle encircling the mouth is the orbicularis oris, which functions like a sphincter to close and pucker the lips. Several groups of muscles insert directly into the commissure, pulling the corner in various directions. The zygomaticus major and levator anguli oris pull the corner upward and laterally, which is fundamental to smiling. Conversely, the depressor anguli oris pulls the corner downward, contributing to frowning and expressions of displeasure.
Common Conditions Affecting the Corners
The unique structure of the oral commissure makes it prone to certain localized skin conditions. The most recognized issue is Angular Cheilitis, also called perleche or angular stomatitis, which manifests as painful, red, and cracked sores. This inflammatory response is often triggered when saliva collects at the corner, creating a moist environment that leads to dryness and cracking. This damage allows microorganisms like Candida yeast or Staphylococcus bacteria to cause an infection. Nutritional deficiencies, particularly in B vitamins or iron, are known risk factors. Other contributing factors include ill-fitting dentures, chronic lip-licking, and conditions that cause skin folds, such as aging.
Role in Facial Expression and Speech
The oral commissure is a focal point for non-verbal communication and articulation. The precise positioning of this corner is instrumental in conveying human emotion; even subtle movements signal amusement or discontent. The commissure’s mobility is also integral to the production of clear speech sounds. It helps shape the oral cavity to control the airflow and resonance needed for proper articulation, especially for vowel sounds and certain labial consonants. The ability to rapidly change the shape and aperture of the mouth allows for the speed and clarity characteristic of human language.