ANATOMICAL PECULIARITIES OF TEETH. 209 



ditions, makes up a large proportion of the tooth in 

 some whales. This identity of tissue, arising in con- 

 sequence of disease, in one animal with that which is 

 normal in another, called forth the remark from Goodsir, 

 that it " must be looked upon as another instance indi- 

 cating the existence of a system of laws regulating the 

 relations between healthy and morbid tissues." 



In this connection a short reference to a curious 

 affection of the teeth of whales may be made. In the 

 grampus and sperm-whale the teeth consist of a hollow 

 cone of dentine coated by cementum ; the pulp chamber 

 is very large and contains a quantity of osteo-dentkie. 

 In one species, Orca, Eschricht found a complete covering 

 of enamel to the free portion of the tooth. Ira Orca, 

 as in the porpoise, the teeth interdigitate when the 

 mouth is closed, and as the teeth wear, owing to the 

 friction, their broad bases come more into apposi- 

 tion. This wearing of the contact surfaces must, in 

 the long run, inevitably open up the pulp chamber; 

 as a rule it is prevented in consequence of the formation 

 of osteo-dentine by the pulp. In some cases the de- 

 velopment of osteo-dentine does not take place with 

 sufficient rapidity, and the pulp becomes exposed and 

 subsequently destroyed. A case of this nature has been 

 minutely described by Eschricht and by Tomes. 



No order of mammals exhibits so many deviations 

 from the ordinary conditions of teeth as whales : not 

 the least interesting abnormality in connection with our 

 present purpose is the remarkable overgrowth of the 

 teeth of Mesoplodon, secured by the naturalists of the 

 Challenger Expedition, and carefully described by Pro- 



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