The observations which have been made respecting the constant growth of the incisor teeth of the glires will apply also to the tusks of the elephant. These are hollow internally, through the greater part of their length, and the cavity con tains a vascular pulp, which makes con stant additions of successive layers, as the tusk is worn down. One of the ele phants at Exeter Change is said to-have nearly bled to death from a fracture of the tusk, and consequent laceration of the vessels of the pulp. The tusks of the hippopotamus, and probably all other teeth of this description, grow in the same matter. Farther and more accurate ob servation may hereafter chew, that the same mode of growth obtains also in other classes of teeth, when they are ex posed to great friction. Something simi lar may certainly be observed in the grin ders of the horse. The tooth is not finish ed when it cuts the gum.: the lower part of its body is completed, while the upper part is worn away in mastication ; and the proper fang is not added till long after. Hence we can never get one of these teeth in a perfect state, for if the part out of the gum is complete, the rest of the body is imperfect ; and there are no 'fangs : on the contrary, when the fangs are formed, much of the body has been worn away in mastica tion. Blake also asserts, that this struc ture is found in the grinders of the beaver, (p. The narwhal is particularly distinguish ed by its long and spiral tusk. The ani mal is found so constantly with only one tusk, that it has been called, in common language, the sea-unicon ; and Linnaeus has even given it a similar appellation, that of monodon. Yet there can be no doubt that it possesses originally two of these ; one in either jaw bone : and that which is wanting must have been lost by some accidental circumstance, as we can easily suppose, (" Shaw's Zoology," vol. ii. p. 473.) These tusks often equal in length that of the animal's body : which may be 18 feet or more : yet they are .always slender.
In many baboons, and most-particular ly in the larger predaceous mammalia, the canine teeth are of a terrific size ; in the latter- animals, the whole profile of the anterior part of the cranium forms a continuous line with these teeth ; which is very visible in the tiger. The canine tusks of the babiroussa, which are very long, and recurved so as nearly to de scribe a complete circle, present the most curious structure. Their utility to the animal appears quite obscure, when their length, direction, and smallness, are considered.
The distribution of the enamel and bony substance varies in the teeth of different animals, and even in the different orders of teeth in the same animal.
All the teeth of the carnivora, and the incisors of the ruminating animals, have the crown only covered with enamel, as in the human subject. The immense fossil grinders of the animal incognitum, or mammoth, have a similar distribution of this substance.
The grinders of graminivorous quadru peds, and the also of the horse, have processes of enamel descending into the substance of the tooth. These organs have also in the last-mentioned animals a third component part, differing in appearance from both the others, but resembling the bone more than the ena mel. Blake has distinguished this by the
name of crusta petrosa ; and Cuvier calls it cement.
The physiological explanation of this difference in structure is a very easy and clear one. The food of the carnivora requires very little comminution before it enters the stomach : hence the form of their grinding teeth is by no means cal culated for grinding : and as the articula tion of the jaw admits no lateral motion, the molares, of which the lower are over lapped by the upper, can only act like the incisors of other animals. The food of graminivorous quadrupeds is subject to a long process of mastication, before it is exposed to the action of the stomach. The teeth of the animals suffer great at.
traten dttring this time, and would be worn down very rapidly. but liar the enamel which is intermixed with their stihstarter, As this pert is hamlet than the other tonstittietits of the teeth. it yids the attrition longer, and presents the appettraitee or eittgre On the worn surface. by which the grinding of the 1ted is moth facilitated..
The distinction or the three solistonees is seen better in the tooth attic elephant than in any animal, The best method or displaying it is by making N longtottlitial vertical section, tool by polishing the tot surfate. The crusts pelettleg will then be tlistilignished bye greater yellowness and tweedy in its rulottr, and by an 0 iditirmi. ty in its tipitearaiite. Its no bulimic or ti. bre:area be distinguished.
The pop or grinding tooth of it gra. amtvorois quadruped is divided into terrain etinital procevies. w kith are unit. tit at their bases, These vary trout two to six in the home Kiel tine, On three the bone of the tooth is Rimed, hh on the tee pulp of the human sottject, but it is here divided into as mail separate *liras as there are prAlvtOte pulp t all of them however enekkied in a remount cep. sole. The tissitloahou etimnitu,es. cc it all teeth, on the points of the little, and extends towards thr basis t when tt has arrived there, the shells unite together' J and they also at their outer margins. lletWeen the processes of the pulp other productivity, it from the capsule in • tinittitt,r du' ,tilt on the gltrfACO of the shells enamel. distinguistia. ble by its tryst:1line apP!lrarauris, toil brutr denominated by Make torte!: stria. tits. When these tnembraillikei produe. lions have (brined their portions of CIA. Mel, they secrete the trusts ptteeeit with. in the ter Wet kit between these prodneti lions of enAmet. The outer maitre or the hone of the teeth It covered by enamel. which may' be to/livererl to that ninth invests the crown or the hit. Mali tooth, e evert that it it tlerteltell im to irregular waving line. in order to ref. der the mill:tee better eideulated fir grinding t end the inetpuilities alio% stir lane or round are tilled op by crusts pelota:, The exterior enamel. and Meta petreee, (Willett may be ate named. by way tif distinguishing them from the rote toes within the tooth.) ere termed by the Mir. Care capsule, If then we make a transverse section of* grinding tooth el' duo horse or row, the exterior nitrate will be (bond to eon.