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PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



an intermediate type continued in a more generalized 

 form, including man. The diagram shows these facts : 



< Carnivorous. 

 . Monkey, man, etc. 

 Herbivorous. 

 Structure of Herbivorous Molars. — The most 

 interesting examples of masticatory teeth are found 

 in herbivores. In these we have veritable upper and 

 nether millstones. Fig. 186 is a face view of a molar 

 of a horse. The double lines are enamel, the shaded 

 spaces dentine, and the unshaded cement. On account 

 of its greater hardness, the enamel stands out as ridges 

 above the softer dentine and cement, and continues to 

 do so however much the tooth wears. 



Fig. 186. — Grinding face of 

 a horse's molar. 



Fig. 187. — Incisor of a horse : 

 A, vertical : B, cross section. 



Origin of this Structure. — The dentine is secreted 

 by the tooth pulp, the enamel by the membranes of the 

 tooth sac. Therefore the tooth sac must have followed 

 the enamel in all its windings. The complexity of 

 structure is therefore the result of infoldings of the sac 

 on the side and down- dippings of the same from above. 

 The cement is afterward formed on the outside, and, as 

 it were, poured over all, filling up the inequalities. A 



