IVORY (FR., Moire, GER., Elfenbein).
The term " ivory " is properly restricted to that substance which forms the main body of the long, projecting, horn-like teeth, called "tusks," of the elephant, and the other prohoscidian quadrupeds belonging to the two genera, Elephas and Mastodon. It is the only form of dentine, or tooth-substance, which, in transverse sections or fractures, shows lines of different colours, or strips, proceeding in the arc of a circle, and forming, by their decussations, minute curvilinear lozenge-shaped spacca. This peculiarity extends to the smallest fragment of true ivory, whether recent or fossil, and servea to distinguish it from all other kinds of tooth-substance, from bone, and from artificial compounds such as celluloid. The large size, and the density of the principal substance, of the teeth of many other speciea of animal, however, favour their application to many purposes analogous to those for which true ivory is used ; these will find a place in the preaent article.
The economic value of teeth is dependent chiefly upon the laws of their growth. Those of limited growth, and which are incapable of renewing the waste that they suffer by wear, as in the case of man and most animala, are practically valueless ; but those which continue to grow as long as the animal lives, as the tusks of the boar, hippopotamus, walrus, narwhal, elephant, and mastodon, are important objects of commerce. In teeth of unlimited growth, tooth-substance is formed at the base as fast as it is worn away from the apex, and thus the growth ia uninterrupted. At first, the ratio of addition is greater than that of abrasion, and the tooth not only grows but increases in aize ; when the animal has attained its full size, however, the tooth ia reproduced without increase of size, or augments only iu length, and that solely where its surface is not abraded by an opposite tooth. The shape of the tooth or tusk, and the impressions on its aurface, are due to the shape of its aocket ; malformations of the latter produce various abnormal forma of tusk. The tusk of the elephant is alightly movable in its socket, and may be readily made to grow in any particular direction by habitual pressure.
Having thus discussed the subject in its general bearings, it will be convenient to devote some separate consideration to each of the ivory-yielding animals—the elephant, mammoth, hippo potamus, walrus, narwhal, and dugong.
elephants are divided into two distinct species—Elephas indicus, found, in aeveral varieties, in both continental and insular Asia ; and E. africanus, widely dis tributed in Africa. The most-important characteristic of the Asiatic elephant, in connection with this article, is that tusks of a size to possess any commercial value are confined to the malea. In the African species, the tusks of both sexes are of marketable growth, though the male is con siderably the larger.
The two large permanent tusks of the elephant are preceded by two small deciduous ones, which make their appearance beyond the gum between the 5th and 7th months. They measure about 2 in. in length, and j in. in diameter where they protrude ; they are abed between the 1st and 2nd years. The permanent tusks cut the gum when about 1 in. long, and at 1-2 months after the " milk-tusks" are shed. At this period, the permanent tusks are black and rugged at the enda ; as they grow beyond the lip, they are worn smooth by the motion and friction of the trunk. The microscopic atructure of the peculiar modification of dentine called ivory is characterized partly by the minute size of the tubes, which, at their origin from the pulp-cavity, do not exceed rcouo in. in diameter ; in their close arrangement, at intervals but little greater than the breadth of a single tube ; and, above all, in their strong and almost angular gyrationa, which are much greater than the secondary curvatures of the tubes of ordinary dentine. By the minuteness and close arrangement of the tubes, and especially by their strongly undulating secondary curves, is produced a tougher and more elastic tissue than results from their disposition in ordinary dentine. Hence the superior value of ivory. Domestication of the elephant is usually attended by depre ciation in quality, and decrease in size, of its ivory ; and scientific observation has not resulted in the discovery of any means for increasing the growth, nor improving the quality, beyond the straightening process alluded to.