186 A MANUAL OF DENTAL ANATOMY. 



one another at considerable intervals ; and by additions to 

 its exterior surface by which it is thickened and strengthened. 

 The study of the growth of the jaw in vertical depth is also 

 very instructive. We find that, as has already been men- 

 tioned, the history of that part of the jaw which lies below 

 the inferior dental canal is very different from that which 

 lies above. From the time of birth to that at which the 

 temporary teeth begin to be cut, the jaw below that line 

 has been making steady but slow progress in vertical depth ; 

 the alveoli, above that line, have been far more active but 

 far more intermittent in their development. 



Again, passing from the nine months foetus to the seven 

 years old child, in whom the temporary dentition is complete, 

 the framework of the jaw below our imaginary line has 



f years 



attained to a depth almost equal to that which it is seen to 

 have in an adult ; in the adult again it corresponds pretty 

 well with that in an aged jaw. The alveolar portion, how- 

 ever, is far deeper in the adult than in the child (this 

 difference is not sufficiently well marked in the figure), and 



(!) Lower jaw. The horizontal line marks the level of the inferior 

 dental canal. 



