DENTITION. 115 



system of operations for giving them this extraordinary- 

 density, which no gradual consolidation could have impart- 

 ed. The formation of the teeth is, in some respects, analo- 

 gous to that of shell; inasmuch as all their parts, when once 

 deposited, remain as permanent structures, hardly ever ad- 

 mitting of removal or of renewal by the vital powers. Un- 

 like the bones, which contain within their solid substance 

 vessels of different kinds, by which they are nourished, mo- 

 dified, and occasionally removed, the closeness of the texture 

 of the teeth is such as to exclude all vessels whatsoever. 

 This circumstance renders it necessary that they should ori- 

 ginally be formed of the exact size and shape which they are 

 ever after to possess: accordingly, the foundation of the teeth, 

 in the young animal, are laid at a very early period of its 

 evolution, and considerable progress has been made in their 

 growth even prior to birth, and long before they can come 

 into use. 



A tooth of the simplest construction is formed from blood- 

 vessels, which ramify through small masses of a gelatinous 

 appearance; and each of these pulpy masses is itself enclosed 

 in a delicate transparent vesicle, within which it grows till 

 it has acquired the exact size and shape of the future tooth. 

 Each vascular pulp is farther protected by an investing 

 membrane of greater strength, termed its capsule^ which is 

 lodged in a small cavity between the two bony plates of the 

 jaw. The vessels of the pulp begin at an early period to 

 deposite the calcareous substance, which is to compose the 

 ivory, at the most prominent points of that part of the vesi- 

 cle, which corresponds in situation to the outer la3'er of the 

 crown of the tooth. The thin scales of ivory thus formed 

 increase by farther depositions made on their surfaces next 

 to the pulp, till the whole has formed the first, or outer 

 layer of ivory: in the mean time, the inner surface of the 

 capsule, which is in immediate contact with this layer, se- 

 cretes the substance that is to compose the enamel, and tie- 

 posites it in layers on the surface of the ivory. This double 

 operation proceeds step by step, fresh layers of ivory being 



