27 



the reader will observe two patches enclosed by white 

 lines. These are the representatives of the two in- 

 fundibula which exist in all of the molars of the 

 upper jaw, but which are not developed in the lower 

 teeth. The similar direction of the lines will enable 

 the reader to connect them with the crusta petrosa, 

 which substance, save in the newly-cut tooth, gene- 

 rally fills up these cavities, although the enamel 

 lining extends almost to the root. The intricacy of 

 the arrangement is, by this diagram, made apparent, 

 and the purpose rendered plain. Without such a 

 provision, the first meal of the animal consumed 

 would probably shatter the instruments of mastica- 

 tion into innumerable atoms ; but girded round, and 

 bound up with the tough and resistant crusta petrosa, 

 however hard may be the fibre the animal chews — 

 however much of grit or sand may be contained in 

 it — or however great may be the force by which the 

 molars are pressed together, and made to pass at 

 the same time from side to side — not a particle of 

 the brittle enamel is fractured. Like a thin layer of 

 glass, guarded between two pieces of wood, it per- 

 forms its office in security ; and as the necessity for 

 the provision, towards which I have directed atten- 



