MICROSCOPIC OBJECTS. 65 



deposited in the form of long prisms, each about -g-^Vo*^ 

 of an inch in diameter, produced in animal cells, the 

 latter being almost obliterated when the tooth is fally 

 formed. In human teeth a coating of enamel is formed 

 over the crown of each. In the teeth of some animals 

 the enamel is disposed in vertical layers among the other 

 structures of the tooth ; this is especially the case with 

 the grinding- teeth of large herbaceous animals. The 

 hardness of the enamel is such that it will in some cases 

 produce sparks of fire like fimt when struck together. 

 This fact is not a little remarkable considering the chemi- 

 cal composition of the enamel, and is one among many 

 examples of the vast changes produced by the living prin- 

 ciple upon matter.-; — Enamel depolarises Ught. 



2. Dentine, or Ivory — ^forms the principal body of which 

 teeth are composed; indeed in some instances, as the 

 tusk of the elephant, no enamel is present. The amount 

 of animal gelatine in ivory is often very considerable. 

 The earthy matter is usually deposited in the form of fine 

 branching cylindrical tubules radiating from the centre of 

 the tooth; on the ends of these dentine tubules the ends 

 of the enamel prisms are placed in the human teeth. 

 Ivory is now established by Professor Owen as an ossifi- 

 cation of the pulp of the tooth. 



3. The Bone, or Cementum, of Teeth — is composed in 

 man of a mass of earthy matter and cartilage, having 

 minute cavities or bone-corpuscles and calcigerous canals. 

 In the molar teeth of the cat-fish, the cavities are large 

 and the canals are. wanting. 





