PERMANENT INCISORS. 



17 



The anterior lace, which is widest above, is liat tVoiii side 

 to side and convex from above to below. It frequently h 

 grooved by a little canal, whicli is most distinct on the free por- 

 tion of the tooth. The incisors of the upper jaw are more con- 

 vex than those of the lower jaw, and tliey frequently have two 

 little longitudinal canals instead of one. The posterior face is 

 convex and rounded from side to side and "Concave from above; 

 to below. The internal border of the tooth is thicker than tlie 

 external ; this is more marked in the imbedded portion than in 



Fig. S. 



I. Longitudinal section of a permanent incisor, leveled. II. Eccentric and 

 longitudinal section of superior and inferior intermediate permanent incisor, to 

 show the differential characters of the dental cup. 



the free portion. The free portion, or crown, is flattened from 

 in front to beliind, and is hollowed out by a cavity known as 

 the external dental cavity or cujk (Fig. 7, B and C.) In tlie 

 virgin tooth tliere is a border in front and a border behind ; the 

 latter is not so high as the first, but in old teeth these become 

 worn off until perfectly level, when the extremity of the tooth 

 takes the name of the fable of the tooth. (Fig. 8, I, upper 

 extremity.) 



The external dental cavity (Fig. 9, <( a a), surrounded by 



