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 „ 0 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 stria, 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 trite 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 -th-,th 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 tuhuli 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 outer layer of cement is relatively thicker than in the tusks of the Mammoth or in those of the Indian elephant. The general cha racter of the microscopic structure of the ivory of the Mastodon's tusk is the same as that of the elephant. The peripheral ex tremities of the dentinal tubes are, in some parts of the tusk, straighter than in the rest of their course ; the straighter extremities were those which were first formed in the calcification of the peripheral part of the pulp, and this first-formed ivory is accordingly, in such parts, more like the ordinary dentine, and is analogous to the thin peripheral cap of such substance in the teeth of the Sloth and of some fishes.
The pulp soon, however, becomes subject to that modification of the calcifying processes by which the more tortuous disposition of the tubuli and the more frequent interposition of opaque cellules are produced ; modifications which, in establishing the characters of ivory, present a step in the transition from true dentine to osteo-dentine.
By the minuteness and close arrangement of the tubes, and especially by their strongly undulating secondary curves, a tougher and more elastic tissue is produced than results from their disposition in ordinary dentine ; and the modification which distinguishes " ivory " is doubtless essential to the due degree of coherence of so large a mass as the elephant's tusk, projecting so far from the supporting socket, and to he frequently applied in dealing hard blows and thrusts.