Introduction 



Next we have tuber culate teeth, with a flat crown from 

 which arise rounded or pointed tubercles; such are many molars 

 and premolars. Besides these there are the flat-topped teeth of 

 horses, cows, elephants and many mice with tortuous ridges 

 across their surface, these being the most complicated teeth known. 



Sections of Teeth. 



I An incisor or tusk of Elephant, with open pulp cavity at base. 2 Human 

 molar with broad crown and two roots. 3 Molar of Ox, showing deeply folded enamel 

 surface with cement filling up the depressions. (After Lydekker). 



A tooth grows from a soft "pulp" and in its early stage 

 is open at the base, the cavity being occupied by the pulp. 

 Some teeth remain this way and continue to grow on indefi- 

 nitely while they wear away more or less at their tips. Such 

 are the tusks of elephants and the incisor teeth of rats and other 

 gnawing animals. Other teeth, on the contrary, gradually close 

 up at the base, forming one or more roots or fangs, the rem- 

 nant of the pulp being contained in the inside of the tooth. Such 

 teeth do not increase in growth after the roots are formed. 



The substances that make up teeth are three: (i) dentine 

 or ivory which forms the bulk of the tooth, (2) enamel, a very 

 hard bluish-white substance which covers the outer surface, 

 and (3) cement, a bone-like substance which fills up the cavities 



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