THE TEETH 269 



over the crown before the tooth is erupted, as in ungulates. 

 After the tooth has been erupted and projects above the gums, 

 it is subjected to wear, and its different constituents become 

 worn away according to their softness. The hardest substance 

 is the enamel, and next comes the dentine, and lastly the 

 cement which is the softest. The result of the unequal wear 

 in teeth like those of the elephant or of some rodents is that the 

 crown is not smooth but becomes ridged like a file, and such 

 teeth are as efficient as mill-stones grinding against one another. 

 In some of the Edentates (sloths and armadilloes) the teeth 

 have no enamel, while in others (ant-eaters) and in some whales 

 there are no teeth at all. 



There is no difficulty in tracing the teeth of vertebrates 

 back to the denticles of the Selachians, and of some of the 

 Ostracoderms. It has been suggested that denticles also gave 

 rise to dermal bone by fusing together. This is very im- 

 probable. Denticles are composed not of bone but of dentine, 

 which differs from bone in that the cells which secrete it do 

 not remain in it but migrate out. Denticles are often found 

 attached to true scales or dermal bones, but these are developed 

 independently from the denticles. 



It can be said that the dermal bones and scales develop in 

 relation to the denticles, but not from them. 



The so-called teeth of Petromyzon, of Ornithorhynchus, and 

 of the tadpole of the frog are epidermal horny structures, and 

 have nothing whatever in common with the true teeth. 



Literature 



Bolk, L. Odontological Essays. Journal of Anatomy. Vol. 55, 1921 ; 



Vol. 56, 1922 ; and Vol. 57, 1922. 

 Mummery, J. H. The Microscopic Anatomy of the Teeth. Henry 



Frowde and Hodder & Stoughton, London, 191 9. 



Tomes, C. S. A Manual of Dental Anatomy, Human and Comparative, 

 Churchill, London, 1808. 



