420 CHAPTER 58. 



P'vaduul, alteraliour^ wliidi (alec place in tlic form of the Iceth and tluMi* 

 l)osition — these latter, however, ai'e more reliable than the marks. 



A satisfactory explanation of the mark cannot, we are afraid, be given 

 without entering at some length into the structure and organisation of 

 the teeth. The Mai'k or Infundibulum is a veiy peculiar hollow extend- 

 ing, when the tooth first comes up, about half an inch down the temporary 

 and rather deeper doAvn the permanent incisors. (Figure 18.) 



Teeth practically may be said to consist of two materials, namely, 

 enamel and dentine. Enamel, which is very hard, sharp, and originally 

 of pearly whiteness, covers the outside of the teeth, and also lines the 

 sides and bottom of the hollow or infundibulum. Thus in the tooth, as 

 it originally appears, there are four walls of enamel. The remainder of 

 the tooth consists chiefly of dentine, a substance of considerable, but less 

 hardness than enamel, and more like ivory. A small quantity of crusta 

 petrosa is also found on the outside of the fang extending upwards and 

 overlapping the enamel covering the crown. 



When an incisor first comes up, the hollow affords lodgment for the 

 debris of the food and the juices expressed from it, and therefore soon 

 looks black. As the tooth wears down, the hollow of course disappears ; 

 but the surface of the dentine immediately below the original hollow, 

 being a somewhat soft material, has become stained for some distance 

 down. Thus there is still a black mark. With the further wear of the 

 tooth the stained portion of the dentine wears away, and the " mark " is 

 then said to be out. The mark, as the reader will easily see from this 

 description, is in a constantly changing condition. 



Premising that the time, v\'hieh the mark will take to wear out, will 

 vary to a greater or less degree according to certain circumstances de- 

 tailed hereafter, we shall now endeavour to give some general rules for 

 guidance. 



Between three and five years old all the marks are very plain in all 

 the permanent incisors. (Figs. 3, 4, 5.) At six, the marks are wearing 

 out of the two centre teeth, which came up at three years old. They are 

 plain in the two next, and perfectly fresh in the two corner teeth. 

 (Fig. 6.) 



At seven, the marks have disappeared from the centre teeth, are wear- 

 ing out of the two next, and are distinct and plain only in the corner 

 teeth. (Fig. 7.) 



At eight, the marks have disappeared from all but the corner teeth, in 

 which they are becoming indistinct. (Fig. 8.) 



At nine, the marks are not usually found in any of the teeth. (Fig. 9.) 



For about two years after the mark has disappeared in each tooth, 

 there may still be seen in the form of a star a trace of the enamel which 

 lined the bottom of the original hollow, and vrhich underlies it for some 

 depth. This star of course decreases in size with the wear of the teeth. 

 About twelve or thirteen the last traces of the enamel have usually dis- 

 appeared even from the corner teeth, but it may remain some time 

 longer. 



Many casual circumstances, however, cause a certain dcgi'ee of devia- 



