200 



ANIMAL PHYSIOLOGY. 



[LESSON 57. 



FIG. 325. 



and such an accident befell this illustration, but which was subse- 

 quently repaired. Here the enamel forms partLy the outside of the 

 tooth, but a long, tortuous, zigzag portion also runs through the centre 

 (a, a). The arrangement of the enamel naturally divides the tooth 

 into four portions ; within each system of enamel is the dentine 

 (>, 5), with pulp cavities (c, c) in the centre. In addition, how- 

 ever, is a layer of crusta petrosa (d, d), which forms the bond of 

 union, or cement, by which the four distinct portions of the tooth are 

 united into one. 



853. By far the most curious arrangement 

 is the molar tooth of the Musk-rat ; the tooth 

 is long, and formed of a reduplication of 

 parts, with the exception that the enamel is con- 

 tinuous from one end of the tooth to the other, 

 bounding the outer surface, and penetrating the 

 various sinuosities. Everywhere within the 

 enamel is the dentine, and, as in other Rodents, 

 the crusta petrosa lies in the centre of the ivory. 

 Cavities formed by the curious direction taken by 

 the enamel, are filled up with crusta petrosa, for 

 the double purpose of cementing this compound 

 tooth firmly together, and for preserving and 

 forming the outlines. A figure of this tooth is 

 given (Fig. 325) ; the enamel (a, a) is seen 

 everywhere surrounding the dentine (5), in the 

 centre of which is the crusta petrosa (c, c), 

 which, filling up the external cavities, is seen 

 at d, d. If the section be made low enough from the surface, the 

 pulp cavities appear. 



Transverse section molar, 

 Musk-rat. 



LESSON LYII. 



THE STRUCTURE OF THE TEETH, CONCLUDED. 



854. The teeth, hitherto examined, have presented all the tissues 

 characteristic of these organs in the higher animals, subject to a varied 

 disposition of the order of arrangement. Teeth are liable to de- 

 generation ; to a loss of tissue, and to a much altered form of the 

 tissue that remains. Of all these structures enamel is found to be the 

 least constant, and, when absent, its place is supplied by a dentine 

 (ivory) of superior hardness. 



