184 A MANUAL OF DENTAL ANATOMY. 



of the alveoli of the central incisors, a position which they 

 afterwards hold with regard to the permanent incisors. The 

 upper of the two pairs of processes are about at the same 

 general level as the mental foramen. 



The general result arrived at by measurements taken 

 from these fixed points is that the alveolar arch occupied 

 by the teeth which have had deciduous predecessors, namely 

 the incisors, canines, and bicuspids, corresponds very closely 

 with the whole alveolar arch of the child in whom the tem- 

 porary dentition is complete ; and that the differences which 

 do exist are referable, not to any fundamental alteration in 

 form or interstitial growth, but to mere addition to its 

 exterior surface. Or more briefly, that the front twenty of 

 the permanent succeed vertically to the places of the tem- 

 porary teeth, the increase in the size of the jaw in an adult 

 being due to additions at the back, in the situation of the 

 true molars, and to other points on the surface. 



If measurements be taken across between the inner platen 

 of the alveoli on either side at the points where they are 

 joined by the septa between the first and second temporary 

 molars, and at about the level of the genio-hyo-glossu& 

 tubercles, it will be found that the increase is slight, if any, 

 notwithstanding that in other dimensions there is a very 

 great difference between the jaws of a nine months foetus 

 and of a nine months child. 



Again, if an imaginary line be stretched across between 

 these two points, and from its centre a line be drawn for- 

 wards to the spina mentalis in the same two jaws, this will 

 be found to differ but little in length in the two specimens. 



But, if instead of measuring to the spina mentalis, the 

 line had been carried to the anterior alveolar plate, a great 

 difference would have been observable ; in point of fact, con- 

 temporaneously with the development of the crypts of the 

 permanent teeth inside them, the temporary teeth and their 

 outer alveolar plates are slowly pushed outwards, a process,. 



