1.56 THE VITAL FUNCTIONS. 



Unlike the bones, which contain within their 

 solid substance vessels of different kinds, by 

 which they are nourished, modified, and occa- 

 sionally removed, the closeness of the texture of 

 the teeth is such as to exclude all vessels what- 

 soever. This circumstance renders it necessary 

 that they should originally be formed of the 

 exact size and shape Avhich 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 deposit the calcareous substance, which 

 is to compose the ivory, at the most prominent 

 points of that part of the vesicle, which corres- 

 ponds in situation to the outer layer of the crown 

 of the tooth. The thin scales of ivory thus 

 formed increase by further depositions made on 



