28 



AGE OF THE DOMESTIC ANIMALS. 



the free portion or crown of these teeth, which becomes worn by 

 friction with the hard substances which the animal takes as food, 

 and by the constant contact wdth the teeth of tlie opposite jaw. 

 After tlie high anterior and lower posterior borders of the virgin 

 incisors have been worn away the table is estabhslied. At first 

 this consists, on each tooth, of an oblong plate, wider from side 

 to side, which is surrounded by the layer of cement directly 

 covering the peripheral layer of enamel ; hiside of this is found 

 the zone of yellowish and softer dentine, usually somewhat de- 

 pressed on account of its lesser resistance to friction ; in the 



Transverse section of an incisor tooth. Inferior, ri;4ht siile. A, anterior; 

 B, posterior ; E, periplieral enamel ; E', ce'.itral enamel ; C, peripheral cement ; 

 C', central cement ; I, ivory, or dentine. (Enlarf^ed.) 



middle, the central enamel or border of the cup surrounds the 

 variable quantity of cement which may be deposited in it. 

 (Fig. 10; Fig. 12,^; Fig. 15.) 



As the table gradually encroaches upon the wedge-shaped 

 tooth (Fig. 16), it becomes proportionately narrower in its trans- 

 verse diameter, and as it becomes oval in shape the cup is found 

 nearer the posterior border of the tooth instead of in the centre. 

 The dental star, or dark-colored dentine which takes the place 

 of the papilla, now appears between the cup and the anterior 

 border of the tooth (Fig. 12, P; Fig. 10, B.). A still further 



