870 



PACHYDERM ATA. 



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 lamina; 

 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 lamina 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. Pallas first 



which is peculiar to the Elephant, viz. that its 

 teeth diminish in length at the same time that 

 they are worn away in depth. 



Whilst the exposed part of the tooth is thus 

 worn away, that part of the root which corre- 

 sponds with the portion ground down is re- 

 moved by a very different process. When 

 examined under these circumstances, the roots 

 of the anterior denticles have the appearance of 

 being eaten away as by a kind of caries, so 

 that all the front of the tooth is thus removed 

 when the grinding surface has ceased to be 

 efficient, and the tooth, when about to be shed, 

 is reduced to a very small size, however large 

 it might have been originally. 



The tooth which is in use is therefore per- 

 petually moving forward in consequence of this 

 process, and making room for that which is in 



explained the real mode of the succession of progress of formation in the hinder part of the 

 ^'"^ jaw to succeed it. This latter, in turn, by its 



development assists in pushing the first for- 

 wards, so that it is strictly true that in the 

 Elephant the second set of teeth grows behind 

 the milk set, instead of above or below them, 

 as in other animals (Jig. 479). 



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 

 lie found the detached 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 laminae are first 

 worn 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 



Fig. 479 . 



deeply 



worn 



away, 



the middle lamina 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 winch explains another phenomenon 



The tusks of the Elephant are very different 

 in their structure from the molar teeth, con- 

 sisting of two substances only, the ivory and 

 the enamel. These tusks grow during the 

 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 

 by the intermaxillary bones of the upper jaw. 

 The 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 in 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 from 

 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 



