DEVELOPMENT AND MORPHOLOGY OF THE TEETH. 69 



Eruption of the Teeth. — The eruption of the milk teeth 

 commonly covers a period of eighteen months, beginning in the 

 6th with the lower incisors and ending in the 24th or 

 30th with the 2nd milk molars. The eruption of the 

 permanent teeth occupies a period of about eighteen years, 

 beginning with the 1st permanent molar in the 6th year and 

 ending about the 24th with the 3rd molars. In civilized 

 races the third molars or wisdom teeth frequently remain im- 

 bedded in the alveolus and may give rise to an abscess. The 

 upper wisdom tooth is developed in the posterior border of the 

 superior maxilla, which bounds the spheno-maxillary fissure in 

 front. The growth backwards of the maxillary antrum converts 

 part of the posterior border of the superior maxilla into the 

 alveolar border, thus bringing the wisdom teeth into position 

 (see page 12 and Fig. 11). The inferior wisdom teeth are 

 developed in the alveolus on the inner aspect of the ascending 

 ramus. 



A fourth molar sometimes appears behind the third. A super- 

 numerary incisor or premolar is very rare. The upper lateral 

 incisor may be very small or even absent. If the teeth are too 

 large for the jaw, a not uncommon condition in civilized races, they 

 appear in irregular positions. 



