secondary dentition, the set of 32 teeth that appears during and after childhood and usually lasts until old age. In each jaw they include four incisors, two canines, four premolars, and six molars. The secondary teeth start to develop in the ninth week of fetal life with the thickening of the epithelium along the line of the future jaw. The permanent first molar in the lower jaw calcifies just after birth, the incisors and the canines approximately 6 months later, the premolars during the second year, the second molar at about the end of the second year, and the third molar at about the twelfth year. The secondary teeth erupt first in the lower jaw, beginning with the first molars in about the sixth year and followed by the two central incisors in about the seventh year, the two lateral incisors in about the eighth year, the first premolars in about the ninth year, the second premolars in about the tenth year, the canines between the eleventh and the twelfth years, the second molars between the twelfth and the thirteenth years, and the third molars between the seventeenth and twenty-fifth years. The eruption of each secondary tooth in the upper jaw lags only slightly behind that of the corresponding tooth in the lower jaw. The third molars in many people are badly oriented or so deeply buried in bone that they must be surgically removed. In some individuals, one or all four of the third molars may not develop completely. Also called permanent dentition, permanent teeth, secondary teeth. Compare primary dentition. See also tooth.
