364 ANATOMY OF VERTEBRATES. 



participates in this revolving course, advancing forward with the 

 teeth. The partition between the tooth in use and its successor 

 is perforated near the middle ; and, in its progress forward, that 

 part next the grinding surface is first absorbed ; the rest disap- 

 pearing with the absorption of the roots of the preceding grinder. 



There are few examples of organs that manifest a more striking 

 adaptation of a complex structure to the exigencies of the animal 

 endowed with it, than the grinding teeth of the elephant. We 

 perceive, for example, that the jaw is not encumbered with the 

 whole weight of the massive tooth at once, but that it is formed 

 by degrees as it is required ; the division of the crown into a 

 number of successive plates, and the subdivision of these into 

 cylindrical processes, presenting the conditions most favourable 

 to progressive formation. The fore and most abraded part of the 

 tooth is fitted for the first coarse crushing of the branches of a 

 tree : the transverse enamel ridges of the succeeding part of the 

 tooth divide it 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 elephant's grinder the advantage of the uneven sur- 

 face which adapts the millstone for its office, but, at the same 

 time, secure the constant presence of the most efficient arrange- 

 ment for the finer comminution of the food, at the part of the 

 mouth which is nearest the fauces. 



The central part of the tusk especially near the base of such 

 as have reached their full size, is occupied by a slender cylindrical 

 tract of modified ivory, perforated by a few vascular canals, which 

 is continued to the apex of the tusk. It is not uncommon to find 

 processes of osteo-dentine or imperfect bone-like ivory, projecting 

 in a stalactitic form into the interior of the pulp-cavity, apparently 

 the consequence of the partial inflammation of the vascular pulp. 



The musket-balls and other foreign bodies which are occasion- 

 ally found in ivory, are immediately surrounded by osteo-dentine 

 in greater or less quantity. It has often been a matter of wonder 

 how such bodies should become completely imbedded in the sub- 

 stance of the tusk, sometimes without any visible aperture, or how 

 leaden bullets may have become lodged in the solid centre of a 

 very large tusk without having been flattened. The explanation 

 is as follows : — A musket-ball, aimed at the head of an elephant, 

 may penetrate, at a, fig. 289, the thin bony socket and the thinner 

 ivory parietes of the wide conical pulp-cavity occupying the in- 

 serted base of the tusk ; if the projectile force be there spent, 

 the ball will gravitate to the opposite and lower side of the pulp- 



