140 THE VITAL FUNCTIONS. 



teeth is such as scarcely to admit of any passages 

 for nutrient fluids.* This circumstance renders it 

 necessary that they should originally be formed of 

 the exact size and shape which they are ever after 

 to possess : accordingly the foundations 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, \Yhich 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 in the 

 interior of its cells the calcareous substance, which 

 is to compose the ivory, at the most prominent 

 points of that part of the vesicle, which corresponds 

 in situation to the outer layer of the crown of the 



* Many anatomists have altogether denied the existence of any 

 such channels; while others allow that they exist, but to a very 

 limited extent. So gradual, indeed, as Mr. Owen observes, are the 

 changes from one modification of dental structure to another, from 

 bone to the densest ivory, that the physiologist who believes the 

 teeth to be inorganic, would be at a loss to draw the line, and to 

 determine where the vital forces cease to manifest themselves, and at 

 which step in the series of tubular structures the tooth becomes an 

 inert body. Ibid, p. 13, It will be seen from the account given in 

 the text that I have adopted the views of Mr. Owen relative to the 

 formation of the teeth. See Comptes Reudus, Dec. 1839, p. 784. 



