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In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: symphysis menti) or line of junction where the two lateral halves of the mandible typically fuse in the first year of life (6–9 months after birth). [1]
The fibrocartilage of the mandibular symphysis fuses together in early childhood. [10] In the adult, the alveolar and subdental portions of the body are usually of equal depth. The mental foramen opens midway between the upper and lower borders of the bone, and the mandibular canal runs nearly parallel with the mylohyoid line.
Strepsirrhines are also traditionally noted for their unfused mandibular symphysis (two halves of the lower jaw), [87] however, fusion of the mandibular symphysis was common in adapiforms, [92] notably Notharctus. [93] Also, several extinct giant lemurs exhibited a fused mandibular symphysis. [94]
The symphysis of the external surface of the mandible divides below and encloses a triangular eminence, the mental protuberance, the base of which is depressed in the center but raised on either side to form the mental tubercle. The size and shape of the bones making up this structure are responsible for the size and shape of a person's chin.
The unfused mandibular symphysis and constricted roots of the lower postcanines support the placement of Alemoatherium within Prozostrodontia, a clade of cynodonts including mammaliaforms and many of their Late Triassic relatives, from Prozostrodon crownwards.
The mandibular symphysis divides below and encloses a triangular eminence, the mental protuberance, the base of which is depressed in the center but raised on either side to form the mental tubercle. The two mental tubercles along with the medial mental protuberance are collectively called the mental trigone.
Sometimes (e.g. in bony fish), the maxilla is called "upper maxilla", with the mandible being the "lower maxilla". Conversely, in birds the upper jaw is often called "upper mandible". In most vertebrates, the foremost part of the upper jaw, to which the incisors are attached in mammals consists of a separate pair of bones, the premaxillae ...
Growth of mandible depends on the normally developing condyles and the muscle function For example, trauma or infection that affect mastoid, middle ear or the joint will result in ankylosis of temporomandibular joint leading to mandibular micrognathia. [31] Macronagthia; Macronagthia is a condition of abnormally large jaws.