SUCCESSION OF TEETH. STRUCTURE OF ENAMEL. 189 



capable of repairing its structure, is proved by the circumstance that a 

 new layer of hard matter is occasionally thrown out upon a surface 

 which has been laid bare by caries. 



316. In those simple teeth which consist solely of Dentine, the mode 

 of production already described, that of the consolidation of a papilla 

 upon the mucous membrane of the mouth, is all which is requisite. 

 When the formation of the tooth itself is complete, it may remain 

 attached only to the mucous membrane, which is the case in the Shark, 

 or it may grow downwards, by the addition of new dental structure at 

 its base, until it comes in contact with the bone of the jaw. Where it 

 is only attached to the mucous membrane, as in the Shark, it is very 

 liable to be torn away ; but a new tooth, formed from a distinct papilla, 

 is ready to replace it; and this process is continually repeated, the 

 development of new papillae being apparently unlimited. On the other 

 hand, where the root of the tooth comes in contact with the jaw, it may 

 completely coalesce with it. which is the case in many Fishes, the Ha- 

 versian canals of the bones being continued as medullary canals into 

 the dentine ; or it may send long spreading roots into the bone, which 

 are united to it at their extremities. In the classes of Fishes and Rep- 

 tiles (with scarcely any exceptions) the teeth are by no means perma- 

 nent, as among Mammalia ; but new teeth are continually succeeding 

 the old ones. The mode in which these teeth originate, by small buds 

 from the capsules of the preceding, will be understood when the capsular 

 development of all the higher forms of the dental apparatus has been 

 described. 



31T. It is obvious that there is no provision, in the simple calcifica- 

 tion of the dental papilla, for any variations of density, other than those 

 which may result from the different degrees of hardness in the substance 

 of the dentine itself. Now in the teeth of Man and most other Mam- 

 mals, and in those of many Reptiles and some Fishes, we find two other 

 substances, one of them harder, and the other softer, than Dentine ; the 

 former is termed Enamel ; and the latter Cementum or Crusta petrosa. 

 For the development of these, a peculiar modification of the apparatus 

 is requisite. 



318. The Enamel is composed of long prismatic cells, exactly resem- 

 bling those of the prismatic shell-substance formerly described ( 281), 

 but on a far more minute scale ; the diameter of the cells not being 

 more, in Man, than l-5600th of an inch. The length of the prisms 

 corresponds with the thickness of the layer of enamel ; and- the two sur- 

 faces of this layer present the ends of the prisms, which are usually more 

 or less regularly hexagonal. The quantity of animal matter^ in the 

 enamel of the adult is extremely, minute, not above 2 parts in 100; 

 and it is only at a very early age that the true character of the animal 

 structure can be distinctly seen. Of the 98 parts of mineral matter in 

 the enamel, 88 J consist (according to Berzelius) of phosphate of lime, 8 

 of carbonate of lime, and 1 J of phosphate of magnesia. The course of 

 the prismatic cells is more or less wavy ; and they are marked by nu- 

 merous transverse striae, resembling those of the prismatic shell-sub- 

 stance, and probably originating in the same cause, the coalescence of 

 a line of shorter cells, to form the lengthened prism. The Enamel is 



