TEETH. 



927 



parallel to the curve described by that move- 

 ment, the upper being the central portion, or 

 that nearest the pivot, the lower, the pe- 

 ripheral portion : the grinding surface of the 

 upper molars is consequently convex from 

 behind forwards, and that of the lower molars 

 concave : the upper molars are always 

 broader than the lower ones. The bony 

 plate forming the sockets of the growing teeth 

 is more than usually distinct from the body 

 of the maxillary, and participates in this re- 

 volving course, advancing forwards with the 

 teeth. The partition between the tooth in 

 use and its successor is perforated near the 

 middle ; and, in its progress forwards, that 

 part next the grinding surface is first absorbed ; 

 the rest disappearing with the absorption of 

 the roots of the preceding grinder. 



There are few examples of organs that 

 manifest a more striking adaptation of a 

 highly complex and beautiful structure to the 

 exigencies of the animal endowed with it, than 

 the grinding teeth of the elephant. We per- 

 ceive, for example, that the jaw is not en- 

 cumbered with the whole weight of the mas- 

 sive 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 pro- 

 cesses, presenting the conditions most favour- 

 able to progressive formation. But a more 

 important advantage is gained by this sub- 

 division of the tooth ; each part is formed 

 like a perfect simple tooth, having a body of 

 dentine, a coat of enamel, and an outer in- 

 vestment of cement : a single digital pro- 

 cess may be compared to the simple canine 

 of a Carnivore ; a transverse row of these, 

 therefore, when the \\ork of mastication 

 has commenced, presents, by virtue of the 

 different densities of their constituent sub- 

 stances, a series of cylindrical ridges of ena- 

 mel, with as many depressions of dentine, 

 and deeper external valleys of cement : the 

 more advanced and more abraded part of 

 the crown is traversed by the transverse 

 ridges of the enamel inclosing the depressed 

 surface of the dentine, and separated by the 

 deeper channels of the cement : the fore-part 

 of the tooth exhibits its least efficient con- 

 dition 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 it into 

 smaller fragments, and the posterior islands 

 and tubercles of enamel pound it to the pu!p 

 fit for deglutition. The structure and pro- 

 gressive development of the tooth not only 

 give to the elephant's grinder the advantage 

 of the uneven surface which adapts the mill- 

 stone for its office, but, at the same time, 

 secure the constant presence of the most 

 efficient arrangement for the finer comminu- 

 tion of the food, at the part of the mouth 

 which is nearest the fauces. 



With regard to the microscopic structure of 

 the peculiar modification of dentine called 

 " ivory," this is characterised partly by the 

 minute size of the tubes, which, at their 

 origin from the pulp cavity, do not exceed 

 T7 ,jL__th 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 much greater than the secondary 

 curvatures of the tubes of ordinary dentine. 



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 dicho- 

 tomously, at acute angles, and gradually de- 

 crease in size as they approach the periphery 

 of the tusk. 



The characteristic appearance of decussating 

 curved striae, with oblique rhomboidal spaces, 

 so conspicuous on transverse sections or frac- 

 tures of ivory, is due to the refraction of light 

 caused by the parallel secondary gyrations of 

 the tubes above described. The strong con- 

 tour lines observed in longitudinal sections of 

 ivory, parallel with the cone of the pulp-cavity, 

 and which are circular and concentric when 

 viewed in transverse slices of the tusk, are 

 commonly caused by strata of minute opaque 

 cellules, which are unusually numerous in the 

 interspaces of the tubes throughout the sub- 

 stance of the ivory, and by their very great 

 abundance and larger size in the peripheral 

 layers of cement. The close-set lateral branches 

 of the calcigerous tubes unite with the tubuli 

 of the cells. The decomposition of the fossil 

 tusks into superimposed conical layers takes 

 place along the strata of the opaque cellules, 

 and directly across the course of the calci- 

 gerous gyrating tubes. 



The radiated cells of the true cement are 

 larger and more uniform in size and shape ; 

 many of them approach nearer the circular 

 figure than in ordinary teeth ; the long axis 

 of the more elliptical ones is parallel with 

 the plane of the stratum of cement ; their 

 average diameter is ^-gVoth of an inch, and 

 their interspaces sometimes do not exceed 

 that dimension. The cemental tubuli appear 

 from their course, and sometimes from the 

 overlapping of the substances in the sections 

 examined, to be directly continued from the 

 tubuli of the ivory ; but Retzius expressly 

 denies the continuation, and states that the 

 cemental tubes at both the outer and the 

 inner surface of the cement have terminations 

 of less diameter than their middle part. This 

 is exact with respect to the major part of the 

 cement. In that near the base of the tusk I 

 have seen a few vascular canals. The contour 

 lines of the cement are usually wavy, and not 

 parallel with the line of the outer surface of 

 the ivory. 



In the tusks of the Mastodon gigantcus the 



