112 



INTRODUCTION. 



first to make their appearance ; then, on each side, the first grinder ; next, 

 the canine tooth ; and, lastly, a second grinder. And here it is to be 

 observed, that they are grinders with four tubercles, and not bicuspid 

 grinders, which are placed next in order to the canine during this primary 

 dentition ; a fact, which is the opposite of what obtains on the completion 

 of the permanent teeth : this phenomenon appears to result from a 

 general law. 



When the sixth, or eighth year is attained, the phenomena of the 

 second dentition commence by the development of a third molar, stronger 

 than those already noticed, and the strongest of the set about to be 

 formed. Next, all the teeth of the first dentition fall out (their roots 

 being more or less absorbed), exactly in the order in which they 

 appeared, and are replaced by others, stronger and larger, but of the 

 same character ; excepting that the two molars, succeeding the canine, 

 are replaced by false, or bicuspid molars. This operation is completed at 

 about the twelfth year ; soon after which the penultimate molar appears : 

 and, lastly, the back molar, or wisdom-tooth, which may be regarded as 

 affording something like an indication of a third dentition. The time of 

 the appearance of this tooth varies : it seldom cuts the gum till several 

 years after the others, and is occasionally delayed till near the thirtieth 

 year. 



In Man, and most Mammalia, the permanent teeth are formed in cap- 

 sules peculiar to themselves, below the first series ; and their formation 

 is not only begun, but advanced, before the fall of the milk teeth, to such 

 a state, that, as these are successively lost, the vacancy is speedily sup- 

 plied. It is owing, no doubt, in a great 

 measure, to the vis a tergo, produced by 

 the outward pressure of the permanent 

 teeth, that the roots of the milk teeth be- 

 come absorbed, and the crowns finally 

 excluded from the jaws. The annexed 

 sketch (fig. 110) displays the relative situ- 

 ation of the two sets of teeth, previously 

 to the fall of the first. 



In the Quadrumana, the permanent 



View of milk, or deciduous teeth, the permanent teeth do not exactly follow in the track of 



teeth being as yet undeveloped, and lying in their -i-i-i n i i -i 11 



deep alveolar cav,ties.-a, the deciduous series; fc, the deClduOUS SCriCS ; IOr behind each QC- 



the permanent series about to become developed. -j ,1 r> ^ J" 1 



ciduous tooth is a foramen leading to the 



cavity, in which the corresponding permanent tooth is in course of forma- 

 tion ; and through this its crown ultimately emerges, at the same time 

 that it forces out the deciduous tooth, and obliterates all trace of its 

 alveolus. During this process, the jaws, in the Simiae, greatly expand, 

 adapting themselves to the size of the permanent teeth, the canines of 



