ELEPHANT. G4 I 



the extremity of the tusk, the ivory itself presents a modification of 

 the exterior layer which is analogous to cement. 



The characteristic appearance of decussating curved striae, with 

 oblique rhomboidal spaces, so conspicuous on transverse sections 

 or fractures of ivory, is due to the refraction of light caused by the 

 parallel secondary gyrations of the tubes above described. The 

 strong contour 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 extremely minute opake cellules, which 

 are unusually numerous in the interspaces of the tubes throughout 

 the substance of the ivory, and, by their very great abundance and 

 larger size in the peripheral layers of the ivory at the extremity 

 of large tusks give them the character of cement. The close-set 

 lateral branches of the calcigerous tubes unite with the tubuli of 

 these minute cells. The decomposition of the fossil tusks into super- 

 imposed conical layers takes place along the strata of the opake 

 cellules, and directly across the course of the calcigerous 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 g^oth 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 con- 

 tinuation, 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 giganteus the outer layer of 



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