NUTRITION PROPER. 



303 



simple case of the down-dipping is seen in the front 

 teeth of the horse. In this case the down-dipping (Fig. 

 187, A) determines on cross section a concentric arrange- 

 ment of the enamel and cement (Fig. 187, B), which suc- 

 cessively disappear as, by use, the tooth wears to lower 

 and lower level (a b, c d, ef), first 

 the cement, and then the enamel, 

 until only the dentine remains. 

 These changes occur first in the 

 middle front teeth, and then in the 

 side teeth. On this fact is founded 

 the mode of estimating the age of 

 horses by the teeth. 



In the horse and cow the ar- 

 rangement of the enamel plates 

 among the dentine and cement is 

 adapted for side-to-side grinding, 

 but some mammals have fore-and- 

 aft grinding. In these the enamel 

 plates are transverse. This is well seen in rodents (Fig. 

 188), and especially in the elephant. The molar of an 



Fig. 188.— The grinding 

 face of a molar of a paca 

 ( Coslogenys) of South 

 America, enlarged. 



Fig. 189.— Elephant's molar: A, showing side and grinding face ; Bisection 

 showing the plates ; .r s, original surface ; a d, worn to a face as in A. 



