ERUPTION OF THE TEETH. 199 



the temporary teeth : and next they were rebuilt about the 

 necks, to form the sockets, of the deciduous teeth. 



Once more, at the fall of the deciduous teeth, the alveoli 

 are swept away, the crypts of the permanent teeth are 

 widely opened, and the permanent teeth come down through 

 the gaping orifices. 



When they have done so, the bone is reformed so as to 

 closely embrace their necks, and this at a period when but 

 little of the root has been completed. 



Take for example the first upper or lower molars : their 

 short and widely open roots occupy the whole depth of the 

 sockets, and reach respectively to the floor of the antrum 

 and the inferior dental canal. No growth, therefore, can 

 possibly take place in these directions ; the utmost available 

 depth has already been reached, and as the roots lengthen 

 the sockets must be deepened by additions to their free 

 edges. 



It is impossible to insist too strongly upon this fact, that 

 the sockets grow up with and are moulded around the teeth 

 as the latter elongate. Teeth do not come down and take 

 possession of sockets more or less ready made and pre- 

 existent, but the socket is subservient to the position of the 

 tooth ; wherever the tooth may chance to get to, there its 

 socket will be built up round it. 



Upon the proper appreciation of this fact depends our 

 whole understanding of the mechanism of teething; the 

 position of the teeth determines that of the sockets, and the 

 form of the pre-existent alveolar bone has little or nothing 

 to do with the disposition of the teeth. 



During the period of eruption of the permanent teeth the 

 level of the alveolar margin is seen, in a dried skull, to be 

 extremely irregular, the edges of the sockets corresponding to 

 the necks of the teeth, whether they have attained to their 

 ultimate level, or have been but just cut. 



And when temporary teeth have been retained for a longer 



