

THE TEETH. 487 



considerable. In this manner, the whole enamel is eventually deposited 

 around the dentinal layer of the crown, while the enamel organ and the 

 pulp gradually diminish, until the former is represented only by a deli- 

 cate membrane ; and the latter presents similar relations to that of the 

 perfect tooth. As yet there exists no trace of cither fang or cement ; 

 they are not formed till the crown is nearly complete, and the tooth is 

 about to emerge. About this time the pulp undergoes a considerable 

 longitudinal growth, while the enamel organ becomes atrophied ; and 

 upon the newly formed portion only dentine, that of the fang, is de- 

 veloped. The tooth, thus forced upwards, begins to press against the 

 upper wall of its sac, and the firm gum which is closely united with it; 

 in which an independent process of absorption also takes place, and the 

 tooth finally makes its appearance. The gum now contracts around it, 

 and the rest of the dental sac becomes closely applied to the fang, and 

 constitutes the alveolar periosteum. 



The milk-tooth attains completeness : 1, by the addition of the 

 remainder of the fang, and the constant elevation of the crown to its 

 normal length ; and 2, by a deposition which takes place from the sac, 

 now united with the alveolar periosteum, which commenced even before 

 eruption, and by which the cement is applied around the fang, while at 

 the same time the tooth is thickened by internal deposition, the pulp 

 diminishing to a corresponding extent. In teeth with several fangs, the 

 pulp, which is at first simple, divides as it elongates, near its point of 

 attachment, a separate fang being developed around each portion. The 

 eruption of the milk teeth takes place in the following order : central 

 incisors of the lower jaw in the Gth 8th month ; central incisors of the 

 upper jaw a few weeks later ; lateral incisors in the 7th 9th month, 

 those of the lower jaw first; anterior molars in the 12th 14th month, 

 those of the lower jaw first; canine in the 16th 20th month ; second 

 molars between the 20th and 30th months. 



The permanent teeth are developed in precisely the same way as the 

 milk-teeth. Their ossification begins, somewhat antecedent to birth, 

 in the first molar, extends, in the first, second, and third years, to the 

 incisors, canines, and premolars, and finally reaches the second molar ; 

 so that in the Gth 7th year there are 48 teeth co-existing in the two 

 jaws, i. e., twenty milk teeth, and all the permanent set with the excep- 

 tion of the wisdom teeth (third molar). When the shedding of the 

 teeth takes place, the bony partitions which separate the alveoli of the 

 permanent from those of the milk teeth are absorbed, and at the same 

 time the fangs of the latter gradually disappear from below in a manner 

 which is not yet understood. The permanent teeth, whose fangs in the 

 meanwhile have elongated, thus become placed immediately under the 

 loosened crowns of the milk teeth, which finally, as the others protrude, 

 fall out and make way for them. The permanent teeth emerge in the 



