640 UNGULATES. 



deeper channels of the cement : the fore-part of the tooth exhibits 

 its least efficient condition for mastication ; the inequalities of the 

 grinding surface being reduced in proportion as the enamel and 

 cement which invested the dentinal plates have been worn away. 

 This part of the tooth is, however, still fitted for the first coarse 

 crushing of the branches of a tree : the transverse enamel ridges 

 of the succeeding part of the tooth divide the vegetable food 

 into smaller fragments, and the posterior islands and tubercles of 

 enamel pound it to the pulp fit for deglutition. The structure and 

 progressive development of the tooth not only give to the Ele- 

 phant's grinder the advantage of the uneven surface which adapts 

 the millstone for its office, but, at the same time, secure the 

 constant presence of the most efficient arrangement for the finer 

 comminution of the food, at the part of the mouth which is nearest 

 the fauces. 



230. Microscopic Structure. — The peculiar condition of the dentinal 

 tubes in the tusks of the Elephant, upon which the texture of 

 ivory depends, has been well described by Retzius ; it consists in 

 the minute size of the tubes, which at their origin from the pulp- 

 cavity do not exceed lioth of an inch in diameter, in their close 

 arrangement at intervals scarcely exceeding the breadth of a single 

 tube, and above all on their strong and almost angular gyrations, 

 which are so considerable that Retzius does not recognize them as 

 analogous to the secondary curvatures in the dentinal tubes of ordi- 

 nary teeth. (PL 149). 



The dentinal tubes of ivory, as they radiate from the pulp- 

 cavity, incline obliquely towards the pointed end of the tusk, and 

 describe two slight primary curves, the first convex towards that 

 end, the second and shorter one concave : these curves in narrow 

 sections from near the open base of the tusk are almost obscured 

 by the strong angular parallel secondary gyrations. The tubes 

 divide dichotomously, at acute angles, and gradually decrease in 

 size as they approach the periphery of the ivory, where they 

 sub-divide, send off many minute branches, and terminate in a 

 thin layer of the basal substance of a yellowish colour by transmitted 

 light, which divides the ivory from the cement. In sections from 



