ERUPTION OF THE PERMANENT TEETH. 323 



The six posterior (or " superadded ") permanent teeth, that is, the 

 three permanent molars on each side, do not come in the place of other 

 teeth. They arise from successive extensions of the dental groove and 

 enamel-germ carried backwards in the jaw, posterior to the milk-teeth, 

 and named by Goodsir ^'■posterior cavities of reserve.'" 



During the general adhesion of the dental groove occurring at the 

 fifteenth week, the part posterior to the last temporary molar follicle 

 continues unobliterated, and thus forms a cavity filled by an enamel- 

 germ, in the fundus of which a papilla ultimately appears, and forms 

 the rudiment of the first permanent molar tooth ; this takes place very 

 early, viz., at the sixteenth week. The deepest part of this cavity is 

 next converted by adhesion into a sac, which encloses the papilla, 

 whilst its upper portion with its contained epithelium, elongates back- 

 wards so as to form another cavity of reserve, in which, at the seventh 

 month after birth, the papilla for the second molar tooth appears. After 

 a long interval, during which the sac of the first permanent molar and 

 its contained tooth have acquired great size, and that of the second 

 molar has also advanced considerably in development, the same changes 

 once more occur and give rise to the sac and papilla of the wisdom- 

 tooth, the rudiments of which are visible at the sixth year. The sub- 

 sequent development of the permanent molar teeth takes ulace from 

 these sacs just like that of the other teeth. 



Calcification begins first in the anterior permanent molar teeth. Its 

 order and periods may be thus stated for the upper jaw, the lower being 

 a little earlier : First molar, five or six months after birth ; central 

 incisor, a little later ; lateral incisor and canine, eight or nine mouths ; 

 two bicuspids, two years or more ; second molar, five or six years ; 

 third molar, or wisdom-tooth, about twelve years. 



Eruption of the permanent teeth. — The time at which this occurs 

 in regard to each pair of teeth in the lower jaw is exhibited in the sub- 

 joined table. The corresponding teeth of the upper jaw appear some- 

 what later. 



Molar, fii'st ......... 6 years. 



Incisors, central . . . . . . . . 7 „ 



„ lateral ........ S ,, 



Bicuspids, anterior . . . . . . . . 9 » 



,, posterior ....... 10 „ 



Canines . . . . . . 11 to 12 ., 



Molars, second . . . . . . . 12 to 13 „ 



,, third (or wisdom) . . . . 17 to 2.j ,, 



It is just before the shedding of the temporary incisors — i.e., about 

 the sixth year, that there is the greatest number of teeth in the jaws. 

 At that period there are all the milk-teeth, and all the permanent set 

 except the wisdom-teeth, making forty-eight (see fig. 224.) 



During the gi'owth of the teeth the jaw increases in depth and length, and 

 undergoes changes in fonn. In the child it is shallow, but it becomes much 

 deeper in the adult. In the young subject the alveolar arch describes almost the 

 segment of a ckcle ; but in the adult the curve is semi-elliptical. The increase 

 which takes place in the lengtli of the jaw arises from a growth Ijehind the posi- 

 tion of the milk-teeth, so as to provide room for the three additional teeth on 

 each side belonging to the permanent set. At certain periods in tlie growth of 

 the jaws there is not sufficient room in the alveolar arch for the growing sacs of 

 the permanent molars ; and hence the latter are found at certain stages of their 



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