23 

 sufficiently removed to expose a portion of the ena- 

 mel : as the years of the animal increase, the outer 

 coating is almost worn away, and not being repro- 

 duced, little of the crusta petrosa will be found on the 

 nippers of very old horses. The ivory, however, is 

 always nearly on a level with the enamel, notwith- 

 standing the greater attrition which the substance 

 forming the principal portion of the table of the tooth 

 must necessarily endure. A sufficient indentation of 

 the ivory, nevertheless, can be observed, to render 

 prominent the ridge of enamel, and to indicate that 

 the last-named material is endowed with the greatest 

 power of resistance. The enamel, in fact, is as hard 

 as flint, and by striking it against a steel, fire can be 

 drawn forth. The three structures, therefore, vary in 

 hardness, and in an opposite direction they contrast 

 to one another in toughness. The ivory is sometimes 

 fractured, but not frequently. I have never seen the 

 crusta petrosa of a living tooth exhibit such an in- 

 jury ; but the enamel is rarely inspected without its 

 being discovered to be more or less in a ragged, 

 chipped, and broken condition, especially at the an- 

 terior edge of the table of the incisors. 



