The typical mammalian tooth is composed of three kinds of tissue, all differing in structure and hardness and called respectively (1) dentine, (2) enamel, (3) cement. (1) The _dentine_, or ivory, is the indispensable tissue of the tooth; the other kinds may be absent, but never the dentine. Chemically, it is like bone, but the microscope shows that its structure is quite different from that of true bone, being composed of an immense number of fine tubules, which radiate from the “pulp-cavity,” or chamber which contains the blood-vessels and nerves, these entering the tooth through the canals of the roots. The tubules of the dentine lodge excessively fine fibrillæ of the nerve and that is why the cutting into a live tooth is so painful an operation. (2) The _enamel_, which is the hardest of all animal tissues, has a polished and shining appearance and is arranged in a mosaic of microscopic prisms, closely packed together, which in most mammals are solid, but in the marsupials, with some exceptions, are tubular. The enamel normally covers the entire crown of the tooth, but does not extend upon the roots, where its superior hardness would be of no advantage. In several instances, always as a secondary specialization, the enamel does not cover the whole crown, but is arranged in vertical bands, it may be on one side only, or at intervals around the tooth. The scalpriform incisors of the rodents, already alluded to, have the enamel band on the front face of the tooth; the softer dentine behind wears away more rapidly, keeping the cutting surface bevelled, like the edge of a chisel, while the hard enamel forms the sharp edge. In some instances the enamel is absent altogether and the teeth are composed entirely of dentine, as in the elephant tusk. In all the Edentata, such as sloths and armadillos, both living and extinct, that have any teeth at all, the teeth have no enamel, but in some of the fossil forms the place of the missing enamel is taken by a harder dentine and thus the effect of differential hardness is secured. [Illustration: FIG. 47.—Section through a lower molar of the Indian Elephant (_Elephas maximus_). Enamel, heavy black; dentine, white; cement, horizontal lines.]