576 SPECIAL PHYSIOLOGY. 



the crown being protected by enamel, and the fang being covered by the cement, 

 which sometimes passes over the crown also. The microscopic structure of 

 these tissues, however, presents certain minute peculiarities. The tusks of 

 the narwhal, walrus, and elephant, are destitute of enamel, and consist almost 

 wholly of the modification of dentine known as ivory, the surface being at first 

 covered by a thin layer of cement, which becomes worn by use. No enamel 

 exists on the molars of the dugong and cachalot, nor on the teeth of the Eden- 

 tata. In the Quadrumana, and in the Carnivora generally, as in Man, the 

 cement is so thin over the enamel of the crown, as to be almost inappreciable ; 

 but it is thick in Herbivora, and especially so on the molars of the elephant, 

 sloth, dugong, walrus, and cachalot. In the Ruminants, in most Rodentia, 

 and in the Pachydermata, the enamel and the cement are arranged, within 

 the crowns of the molar teeth, in double vertical plates or folds, between cor- 

 responding processes of the dentine, the variations in which form a means of 

 classification in the Rodentia and Pachydermata. 



When one of these compound teeth, such as a molar of the ox, deer, sheep, 

 horse, or the still more complex grinder of the elephant, first cuts through the 

 gum, the crown is covered with a thick layer of cement, which dips in between 

 folds of enamel, which, in their turn, conceal variously-disposed plates of den- 

 tine. In the course of time, the cement on the grinding surface is worn down, 

 and the folds of the subjacent enamel become visible. With further attrition, 

 the cement between the folds of enamel wears away faster than the enamel 

 itself, and hence the broad surface presents ridges corresponding with the 

 harder enamel, and furrows corresponding with the softer cement, an arrange- 

 ment well adapted, like the roughened surface of a mill-stone, for the grinding 

 of hard grain, woody fibre, or roots. As the process of wear extends, the 

 summits or bent parts of the folds of enamel are also worn through, and the 

 concealed plate of dentine is exposed ; in this case, the most complex markings 

 appear on the grinding surface, produced by the alternating and often tortuous 

 bands of dentine, enamel, and cement. 



When the mammalian teeth, whether simple or complex, are worn down to 

 the fang, they generally, as in Man, loosen and fall out ; for their growth is 

 completed at a certain period, after which their pulps shrink, they become 

 subject to wear or decay, and undergo little or no repair. A remarkable pro- 

 vision exists, however, for the preservation of the cutting edge of the chisel- 

 like incisor teeth, characteristic of, and necessary to, the gnawing Rodentia. 

 These teeth show a persistent growth ; the fang is deeply implanted in the 

 jaw, and remains hollow and open at the base, into which the persistent pulp 

 extends. The so-called enamel organ, on the anterior wall of the socket, is 

 also persistent. Fresh dentine is constantly being formed within, upon the 

 pulp, and fresh enamel upon the anterior surface, by the enamel organ ; whilst 

 the unequal wear of the hard coating of enamel in front, and of the dentine 

 behind, preserves, during the whole of life, the chisel-like edge. From the 

 persistent growth of these peculiar teeth, it happens, that if one of them be 

 drawn or accidentally lost, the opposing tooth being no longer worn down by 

 use, continues to elongate, and, following its natural curve, attains an abnor- 

 mal size and shape, and its point turns round, and even penetrates the oppo- 

 site lip. The teeth of the armadilloes and sloths also grow continuously, on 

 persistent pulps. 



In many Mammalia, sex exercises a remarkable influence on the develop- 

 ment of certain teeth. Thus, in the Quadrumana, especially in the anthropoid 

 apes, the upper canine teeth, in the male, are more than twice the size of the same 

 teeth in the female ; the tusks of the boar and of the male elephant, and musk- 

 deer, are larger than those of the female animals. In the dugong, which, an 

 exception in Cetacea, has both temporary and permanent incisor teeth in the 

 two jaws, the upper permanent incisors project beyond the gum, in the male ; 

 but in the female, the permanent incisors in both jaws remain concealed 

 throughout life, their growth being arrested before they cut the gum. The 

 asymmetrical tusk, the rudimentary and concealed condition of the opposite 

 tooth of the male narwhal, and the hidden rudiments of both teeth in the 

 female, already mentioned, also show the influence of sex. 



This rudimentary condition of certain teeth is, however, sometimes hide- 



