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The mandibular symphysis is the area of the lower jaw where the two halves that comprise the mandible join, or articulate. This line of junction is named after the clinical term of the lower jaw, called the mandible. The mandibular symphysis is also known as the symphysis menti.
Symphysis is the term given to the places that join two bones, thus forming a midline. In most cases, the symphysis is amphiarthrotic, meaning that it is slightly movable, which is made possible by an attachment of fibrocartilage. In the case of the mandibular symphysis, however, the bones are completely fused and cannot move at all.
The mandibular symphysis in particular has a faint ridge, which is the result of its formation during one's early years. The feature is found at the mandible's external surface, in the middle of the area that forms a person's chin. The mandibular symphysis separates at its base as it travels downward, enclosing a triangular projection called the mental protuberance. This part of the jawbone has a depressed base with flanking elevations, which create the mental tubercle.
On each side of the mandibular symphysis is an incisive fossa. This feature is so named because it is a depression immediately below the area where the mandible bears the lower incisors, which are the frontmost teeth in humans and are used for cutting food. It serves as the origin of the mentalis, a muscle located at the tip of the chin that is responsible for pushing up the lower lip for facially demonstrating displeasure or doubt. It also bears a small section of the orbicularis oris, a muscle that encircles the mouth and is used for puckering and closing, thus making it essential for playing instruments such as the flute or trumpet.
Two muscles originate at the mandibular symphysis. The geniohyoid is a narrow muscle that runs from the chin to the hyoid bone at the neck's midline. The other muscle, the genioglossus, starts from the chin as well, but terminates at the tongue.
The mandibular symphysis is known as one of the most prominent symphyses in the body. Another major symphysis is the pubic symphysis, which joins the top section of the pubic bones in the pelvis. Another one, the sacrococcygeal symphysis, is also called the articulation of the sacrum and coccyx: named after the parts of the vertebral column that it begins and ends. The term "pelvic symphysis" is given to the combination of the public symphysis and the ischiatic symphysis, which is the fusion point of the lower and back parts of the hip bone.