1120 



THE DIGESTIVE SYSTEM. 



(Fig. 886). This arrangement is brought about largely by the great width ol 



the upper central incisors as com- 

 pared with their fellows of the 

 mandible, which throws the uppei 

 canines and the succeeding teeth 

 into a position behind that of the 

 same -named teeth of the lowei 

 set. But as the lower molars art 

 larger in their antero-posterioi 

 diameter than those of the uppei 

 row and this remark applies 

 particularly to the third molars 

 the two dental arches terminate 

 behind at approximately the sam< 

 point. 



The upper dental arch is said fej 

 form an elliptical, the lower a parabolii 

 curve (Figs. 884 and 885). The Un- 

 formed by the masticating surfaces o 

 the upper teeth, as seen on profile vie\ 

 (Fig. 886), is usually somewhat convex 

 owing largely to the failure of th< 

 third molar to descend into line wit! 

 the others. Similarly the line of th 

 lower teeth is as a rule concave. 



In both jaws the crowns of the fron 



teeth are higher (longer) than those o 

 FIG. 885. HORIZONTAL SECTIONS THROUGH BOTH THE MAX- +u e >_ 



ILLA AND MANDIBLE to show the roots of the teeth. The 



3rd molar 

 2nd molar 

 1st molar 

 2nd premolar 

 1st premolar 



Canine 

 Lateral incisor 

 Central incisor 



::: = : ^ 3rd molar 

 2nd mola 



1st molar 



2nd premolar 



s 1st premolar - 



,, Canine 

 Lateral incisor 

 Central incisor 



Period of Eruption of the Per 



sections were carried through the bones a short distance 



from the edge of their alveolar borders. The upper figure 



shows the upper teeth, the lower figure the lower teeth, manent Teeth. Although then 



Note the flattened roots of the lower incisors, the two j s considerable Variety in th< 



root-canals in the anterior root of each lower molar, and j , v T. jv 



the confluence of the three roots of the upper last molars. " a ^ S at Which the Various pel 



manent teeth appear above th 



gums, the order of eruption is practically constant in different individuals, am 

 is as follows: Before any of the 

 deciduous teeth are lost the first 

 permanent molars appear behind 

 the 2nd deciduous molars. Next, 

 the central deciduous incisors fall 

 out, and their places are taken by 

 the permanent teeth of the same 

 name; then follow the remaining 

 teeth in the following order: 

 Lateral incisors, 1st prernolars, 2nd 

 premolars, canines, 2nd molars, 

 and 3rd molars. It will be ob- 

 served that the eruption of the 

 canine is delayed until the two 

 premolars, which succeed it in the 

 row, are cut, so that it breaks the 

 otherwise regular order of eruption. 

 The 1st molar is sometimes popu- 

 larly known, owing to the date of 

 its eruption as the "six-year-old PIG> 886 ._ To show the relation of the upper To the low. 



tooth, and the 2nd molar as " the teeth when the mouth is closed. The manner in which 



twelve-year-old tOOth " tooth of one row usually strikes against two teeth of 1 



The dates at which the erup- ^tfbVnoI'ed'^ ^ ^^ interlocking 

 tion usually takes place may be 



simply stated as follows for the lower teeth ; those of the upper jaw appear a littl 

 later : 



