270 NATURAL THEOLOGY. 



XI. 



THE SUBJECT PURSUED WITH REFERENCE TO THE 

 FORMATION AND GROWTH OF TEETH. 



Seeing how admirably these instruments have 

 been adapted to their several uses, the reader must 

 be curious to know how they are produced — how 

 they are manufactured with so fine a "prospective 

 contrivance." Three different substances are ex- 

 posed on making the section of a tooth ; viz. ivory, 

 or the bone of the tooth — the enamel, which is 

 very hard, and breaks with a vitreous fracture — 

 and a substance differing in some respects from 

 both, and which English anatomists have called 

 crusta petrosa, and Cuvier cement. These three 

 substances are not formed in every tooth. Some 

 teeth consist entirely of the bone or ivory, as those 

 of the porpoise and bottle-nosed whale. In man, 

 and in the carnivorous animals, the bone is cover- 

 ed with enamel, and in the graminivorous and ru- 

 minant animals, all three substances are found. 



In the chapter on bone, it has been shown that 

 its texture must be loose to admit blood-vessels ; 

 for bone is nourished and undergoes changes 

 through the influence of the circulating blood in it. 

 But the almost stony density of the teeth does not 



