Before considering the different kinds of ivory, and the various uses to which it is put, let us glance at its composition. In structure, it is equivalent to dentine, the material of which nearly all teeth are composed, and has an organic base or matrix, which upon examination is seen to be permeated by a vast number of very small and delicate canals, each of about one-fifteenth of an inch in diameter, which seem to commence at the pulp-cavity, presumably the axis, and extend outward to the periphery of the tusk. The little canals are not packed closely together, but are separated by spaces of about their own diameter. To these tubes, the regularity of their disposition and their delicacy, the ivory owes its fineness of grain, and its remarkable elasticity. By examining them, an expert can distinguish elephant ivory from any other; as they have a peculiarity of making a series of decided bends in their course, from the axis to the periphery, which produces a graining in the ivory, unique and peculiar. Ivory is often confused with bone, but is a very different substance. It is much finer in general structure, much more elastic, and is without the canals that convey blood-vessels through the bones. If a section of a tusk is made some distance from the growing pulp, the centre, or core, will be found to be darker than the rest, and of a different nature. This is the remains of the pulp. The outer portion of the tusk is still different, or composed of a compact layer of cementum that covers or encloses the entire tusk. The intermediate substance is ivory, which shows many circular lines about the central dark spot, calling to mind the growth-marks seen in sections of trees, and due to the fact that in all ivory there are great numbers of very minute spaces known as “interglobular spaces.” The localities occupied by these spaces are characterized by a smaller proportion of lime-salts and a greater proportion of organic matter than other portions. Hence this part of the ivory is not so dense as the rest, and is more liable to decomposition: so, in many of the fossil tusks that are found, a sectional view often shows it separated into six or seven distinct rings, the intermediate organic matter having disappeared. It is supposed, that, in living ivory, these interglobular spaces are filled with some organic substance. According to Von Bibra, ivory contains from forty to forty-three per cent of organic matter; while human dentine contains from twenty-four to thirty-four per cent. From its delicate structure, ivory takes a rich polish; and it is also susceptible of being dyed. The ease with which it is carved, makes it one of the most valuable of all materials for artistic carving.