The membranous prolongations of the cap sule of the tooth are retracted towards the top and towards the sides of the cavity, in propor tion as the cementum which they deposit fills up the spaces between the different lamina of the tooth. The summits even of the laminre are covered with cement= before the tooth begins to be worn. Sometimes, indeed, the same prolongation of the capsule is secreting cementunt near around the top of a lamina, whilst it is forming enamel lovver dovvn. From the same cause, the upper portion of the inter spaces is already filled up with cementum, while the lower parts remain separate, under which circumstances the lower portion of the capsular prolongation becomes separated from the upper, and only receives its nourishment through its lateral adhesions to the capsule.
The deposit of enamel commences alinost at the same time as the formation of the ivory, and the secretion of the eementum speedily follows; so that the summit of each lamina has all the three substances belonging to it com pletely formed long before its base and conti guous laminm are soldered together by their upper portions, even before their bases are completed. We may likewise add that all these different operations are by no means equally in progress in all points of the tooth at the same time, but they occur much earlier in front than behind ; so that the anterior laminw may be already consolidated by their summits and even by their bases while the bases of the middle ones remain separate, and when the posterior laminT are not even formed, or only represented by the patches of ivory that are first deposited upon the apices of the pulps. There was formerly much discussion as to the number of grinding teeth proper to the Elephant, and as early as 1715 the Royal Society of London observed that there is sometimes only one and sometimes two on each side in either jaw, and moreover that the first tooth is longer or shorter in proportion to the second in different individuals. l'al las first explained the real mode of the succession of these teeth, accounting for all these irregu larities, and showing that the Elephant has at first only a single tooth on each side, until a second, developing itself, replaces the first, so that during a certain period there are two, until the shedding of the first again leaves only one. Cuvier first announced that this succession and consequent alternate change in the number of the teeth was repeated more than once, because he found the deGached germs of a third grinder in the jaw of an Elephant, with two teeth in situ.
It thus becomes easy to understand how the grinding teeth of the Elephant, notwithstanding the enormous wear to which they are perpetually subject, are kept constantly ready for use, and renovated in front as fast as they are worn away behind. No sooner has the body of the first formed tooth pierced the gums than it begins to undergo important changes. As the Elephant is herbivorous, its teeth are necessarily worn away by mastication like the teeth of all other herbivorous animals, a circumstance which is indeed necessary in order that the grinding surface may be constantly kept in a condition to bruise vegetable substances. The little in
dentations on the tops of the laminre are first NVOTI1 off, until the wearing down has reached the interior of the tooth, when each denticle of course presents an oval disc of ivory, surrounded by a ring of enamel and enclosed in the ce mentum, three substances, which, being of very different degrees of hardness, are ground away unequally, so as always to present a rough grinding surface like that of a mill-stone.
The tooth, moreover, by its rhomboidal figure and.very oblique position in the jaw, presents its anterior portion above the gums long before the posterior, so that the surface produced by mastication forms an obtuse angle with the plane of the upper surface of the tooth : hence it happens that when the front of the tooth is deeply wom away, the middle laminm are scarcely used at all, and the hinder ones remain quite intact, presenting the summits of the indentations of their crowns under the form of little round eminences. In the same way the anterior denticles are altogether destroyed before the posterior are very far worn down, a circum stance which explains another phenomenon The tusks of the Elephant are very differe in their structure from the molar teeth, col sisting of two substances only, the ivory a the enamel. These tusks grow during th whole life of the animal, and sometimes attain enormous dimensions, measuring eight or nine feet in length, and weighing upwards of two hundred pounds. In the females of the Asiatic Elephant the tusks are very small, but in the African Elephant both sexes have these defences largely developed. These remarkable teeth, which are evidently the representatives of the enormous tusks bestowed on some of the Ceta cea, such for example as the Narwal ( Mono don), are implanted in enormous sockets formed 11 the intermaxillary bones of the upper jaw. 'he central portion of each tusk, the ivory, which forms by far the greatest portion of the tooth, is secreted by an enormous pulp lodged in a deep cavity which is excavated its root, from the surface of which it is deposited layer by layer in successive strata. The pulp or nucleus from which the mass of the tooth is thus formed has not the slightest organic con nection with the ivory which has been the product of its secretion ; not a fibre or vessel, or even the slightest cellulosity passing front one to the other. The tusk is therefore only kept in its socket by the tight embrace of the parts around it, and its direction may be readily changed by gentle and continued pressure,. in the same way as dentists succeed in changing which is peculiar to the Elephant, viz. that its teeth diminish in length at the same time that they are worn away in depth.