PERMANENT INCISORS. 19 



This is sometimes a hollow cup, but it is frequently filled 

 with cement, makino- tlie free extremity, even in the viro-in 

 tooth, in such cases perfectly solid. The cups in the inferior 

 teeth are less deep than those in the superior incisors. 



In the incisors of the lower jaw the pincher teetli have a. 

 cup witli an average depth of 16 to 18 mm. ; the cup of inter- 

 mediate teeth is 18 to 20 mm., and that of the corner teeth is 

 11 mm. to 13 mm. In the upper jaw the cup of the pinchers' 

 is 25 mm. to 27 mm. deep ; that of the intermediate teeth, 27 

 mm. to 28 mm. deep; and that of tlie corner teeth somewhat 

 less, 18 mm. to 20 mm. (Fig-. 9, acta ; Fig. 12, c.) The cups 

 in the lower teeth incline nearer the posterior border of the tooth 

 than in the upper ones. 



In the imbedded portion of the tooth there is a large canal 

 •which holds the dental pulp or papilla, which is known as the 

 internal dental cavity. (Fig. 9, h ; Fig. 12, P.) Examine Fig. 9, 

 a,nd we find that this cavity occupies the centre of the tooth, in 

 the root, but toward the crown inclines toward the anterior face 

 and penetrates between the latter and the cup of enamel. As 

 the tooth becomes older this cavity becomes filled with an ivory- 

 like substance, softer and darker colored than the rest of the 

 tooth, and when it appears on the table of the tooth, from the 

 wearing away of the crown, it is known as the ''dental stary 



In order to understand clearly the various forms which the 

 table of the tooth assumes as it wears away, a series of sections 

 are shown in Fig. 10, representing the table as it appears from 

 the use of successive years. 



A. The rubbing surface of the dental table is at first flat- 

 tened from in front to behind ; that is to say, its transverse 

 diameter is greater than the antero-posterior. (Sections 1 and 2.) 



B. It becomes oval ; there is still a disproportion between 

 the extent of its two diameters, but the transverse diameter 

 remains greater than the antero-posterior. (Sections 3, 4, 

 and 5.) 



C. It becomes rounded and its two diameters are nearly 

 •equal. (Sections 6 and 7.) 



