32 



conjectural function of secretion ; and, in further 

 proof that it does not secrete, it can, in the young 

 subject, before the tooth has emerged from its pri- 

 mary cavity, be separated with the greatest facility 

 from the enamel. It serves to protect the enamel, 

 but it likewise answers the end of keeping the tooth 

 firm in the jaw : around the neck of the molars it 

 becomes thickened to a great extent; and at the 

 roots, especially of old teeth, it exhibits considerable 

 substance. The horse's molars are continually being 

 wrenched from side to side while the food is being 

 ground, and unless they were very firmly mortised, 

 the vessels which nurture them would be continually 

 lacerated : this the crusta petrosa, by entirely filling 

 the alveolar cavity, prevents ; and as the molars 

 wear down with age, the thickening of the mem- 

 brane enables the jaw to retain with security the 

 latest portion of the tooth. I have specimens taken 

 from very aged subjects, where the fang having been 

 worn away, the molar consists merely of the roots of 

 teeth embedded in a mass of crusta petrosa. 



16. The various parts and peculiarities of form, 

 now require consideration. If the table of an incisor 



