36 ON THE ORIGIN AND DEVELOPMENT OF THE 



As tlie sacs increase in size, the alveoli increase also, and when 

 the permanent form slight projections behind the temporary 

 tooth-sacs, niches""' are formed for them in the posterior walls 

 of the alveoli. Whilst this increase in the bulk of the sacs 

 and alveoli is going on, there is no proportionate increase in 

 the length of the jaw, in consequence of which, the sac of the 

 anterior permanent molar has been insinuating itself into, and 

 at the eighth month, or the full time, is almost wholly im- 

 bedded in, the maxillary tuberosity,t and has become situated 

 on a higher level than the milk-sacs, during which it has not 

 only drawn the surface of the gum upwards and backwards, 

 but has also lengthened out the great or posterior cavity of 

 reserve. 



About this time the fangs of the milk-incisives begin to be 

 formed, in the accomplishment of which three contempora- 

 neous actions are employed — viz, the lengthening of the pulp, 

 the deposition of tooth-substance upon it, and the adhesion to 

 the latter of that portion of the inner surface of the sac which 

 is opposite to it. 



While the fangs of the milk-teeth, particularly those in the 

 front of the jaw, are lengthening in the manner now described, 

 the pulps and sacs of the permanent teeth continue to increase, 

 and the bony crypts which contain them to enlarge in pro- 

 portion, the lower edges of the latter insinuating themselves 

 between the two former. As this process continues, the jaw 

 lengthens more rapidly, and when the infant is eight or nine 

 months old, there is so much room in the alveolar arch, that 

 the anterior permanent molar tooth begins to resume its 

 former position in the posterior part of the dental arch, and 

 the great cavity of reserve again to return to its original size 

 and situation. 



About tliis time the central incisives begin to pass through 



* Bell, A'liatomy, etc. of the Teeth, p. 62. 

 t Hunter, Nat. Hist. Human Teeth, pp.- 101, 102, 103. 



