272 NATURAL THEOLOGY. 



process thus begun is completed by the secretion 

 of successive strata from the surface of the pulp, 

 until the bone of the tooth, which was at first a 

 mere scale, becomes a dense body, with a small 

 cavity and tube leading to it ; and the pulp, which 

 was of the size and shape of the tooth, shrinks to 

 a mere shred, containing a nerve and vessels. 



Thus the original use of the pulp is changed : 

 but it remains, serving an important purpose ; for 

 by sustaining a sensible nerve within the tooth, 

 it extends a degree of protection to it. The teeth 

 are sensible to what we masticate ; they are sen- 

 sible to the smallest particle of sand, and so are 

 they to the degrees of heat. This sensibility is 

 necessary to their protection, and to the continu- 

 ance of their vitality, yet the sensibility is not in 

 the substance of the tooth, but in the nerve 

 within ; and the density of texture of the tooth 

 becomes a medium through which both mechani- 

 cal vibration and heat are readily communicated 

 to the nerve. 



We have seen how the bone of the tooth is 

 secreted by the pulp. The enamel is formed 

 differently. The sac, which covers the whole 

 pulp and rudiments of the tooth, has a fine orga- 

 nization, as displayed by the art of the anatomist, 

 and its inner surface throws out a fluid which, 

 falling on the surface of the bone of the tooth, 

 (already formed by the pulp,) there concretes, or 

 ndergoes a species of crystallization, and har- 

 dens into enamel. 



