136 THE VITAL FUNCTIONS. 



surface ; preserving it always in that rough con- 

 dition, which best adapts it for the bruising and 

 comminuting of hard substances. 



The incisors of tlie rodentia are guarded by a 

 plate of enamel on their anterior convex surfaces 

 only ; so that by the wearing down of the ivory 

 behind this plate, a wedge-like form, of which the 

 enamel constitutes the tine cutting edge, is soon 

 given to the tooth, and is constantly retained as 

 long as the tooth lasts (Fig. 280). This mode of 

 growth is admirably calculated to preserve these 

 chisel teeth fit for use during the whole life-time of 

 the animal ; an object of greater consequence in 

 this description of teeth than in others, which con- 

 tinue to grow only during a limited period. The 

 same arrangement, attended with similar advan- 

 tages, is adopted in the structure of the tusks of the 

 Hippopota mus. 



In teeth of a more complex structure, a third 

 substance is found, uniting the vertical plates of 

 ivory and enamel, and performing the office of an 

 external cement. This substance has received 

 various names, but it is most commonly known by 

 that of the cement, or Cnista petrosa. It resembles 

 ivory both in its composition, its extreme hardness, 

 and in tlie circumstance of its intimate structure 

 being composed of minute tubular fibres; but it is 

 generally more opaque and yellow than that sub- 

 stance ; and inasmuch as it contains cells disposed 

 in radiating lines,* it approaches more nearly in its 

 structure to true bone. 



In the pharyngeal teeth of the Scams, or parrot- 



* These radiated cells have been described by Purkinje as being 

 characteristic of true buae. 



