The left and right halves of the lower jaw, or mandible, begin originally as two distinct bones, but in the second year of life the two bones fuse at the midline to form one. The horizontal central part on each side is the body of the mandible. The upper portion of the body is the alveolar margin, corresponding to the alveolar margins of the maxillae. The projecting chin, at the lower part of the body in the midline, is said to be a distinctive characteristic of the human skull. On either side of the chin is the mental foramen, an opening for the mental branch of the mandibular nerve, the third division of the fifth cranial nerve.