LESSON 55.] THE STRUCTURE OF THE TEETH. 



197 



FIG. 320. 



contact with enamel, ivory, and bone, lying beside each other, they 

 will wear unequally ; the softest (the bone) will cut the fastest, and 

 form the deep hollows described ; the next in hardness, the ivory, 

 will wear less than the bone, but more than the enamel ; and lastly, 

 the enamel, wearing but little, will form the high ridges described. 



843. But this is not all : to render the inequalities certain and 

 constant, there are three systems of enamel in the grinding teeth of 

 the upper jaw of the Cow (a, a, a, Fig. 320), and only two in those 

 of the lower jaw, so that enamel 



never opposes enamel ; by all 

 these contrivances the teeth must 

 necessarily wear unequally, and 

 always present a rough, uneven, 

 grinding surface to the food. 



844. To show the arrange- 

 ment of the enamel of a molar of 

 the upper jaw, a figure (320) is 

 given, but slightly magnified. 

 Crusta petrosa (b) is found sur- 

 rounding the tooth ; within this 

 a layer of enamel (a), which 

 passes irregularly round the outer 

 surface ; next to this is ivory, to 

 which succeeds crusta petrosa, on 



the other side ef which ivory again appears, to be bounded by the 

 enamel. Take a given portion of a tooth, and the arrangement of 



FIG. 321. 



Molar of Cow. 



Portion of Molar tooth, Sheep. 

 a, a, Enamel ; &, &, Dentine ; c, Cementum. 



