474 ORGANS OF DIGESTION. 



rule. Owing to this tooth growing at the posterior extremity 

 of the alveolar processes, in a place where, from the preceding 

 development of the other teeth, it is much cramped for room, 

 it is not only imperfectly evolved in most cases, but it often 

 takes a very irregular direction; its grinding surface sometimes 

 looking forwards and sometimes backwards. 



The Alveolar Processes in each jaw form a semi-elliptical 

 row of sockets, for the insertion of the roots of the teeth into 

 them. These processes and the teeth, as Mr. Hunter has very 

 properly explained, have such a mutual dependence upon each 

 other, that the destruction of the one is inevitably followed by 

 that of the other: "If we had no teeth, it is likely we should 

 not only have no sockets, but not even these processes in which 

 the sockets are formed."* The semi-elliptical arrangement ob- 

 served by the teeth is such, that when the mouth is closed, the 

 exterior circumference of the row above projects beyond those 

 below; this is more obviously ihe case in front; but it also pre- 

 vails at the sides, and depends primarily upon the greater 

 breadth of the incisors of the upper jaw. The grinding sur- 

 face of the under row, as a whole, is slightly concave from be- 

 fore backwards, while the opposed surface of the upper row 

 has a corresponding convexity. Each row, viewed collective- 

 ly, forms a single edge, in front; but after having passed the 

 cuspidati, it becomes thicker, forms a double edge, and is con- 

 tinued backwards in that state. 



SECT II. OF THE TEXTURE AND ORGANIZATION OF THE TEETH. 



The teeth consist in two kinds of substance, one of which is 

 ivory or bone-like, and the other enamel. 



. The Enamel forms the periphery of the body of a tooth, and 

 is distinguished by its whiteness, its brittleness, its semi-trans- 

 parency, and a hardness so considerable that it soon takes down 

 the edge of the best tempered saw or file, so that it is very dif- 



* Loc. cit. p. 7. 



