22 



better understood by reference to the accompanying 

 wood-cut, which represents one of those members 

 divided down its centre : — a indicates the crusta pe- 

 trosa, which can be traced to enclose the organ, and 

 to dip down the infundibulum b. The crusta petrosa 

 is shown to be thicker at the root of the fang, and at 

 the base of the infundibulum, than at other parts. 

 b denotes the ivory, which forms the principal and 

 central portion of the tooth, in the middle of which is 

 the darker space marked by the letter d. This last is 

 intended to represent the cavity of the pulp, which, in 

 the young incisor is, as in the diagram, of great length. 

 Between the ivory 5, and the crusta petrosa a, is a 

 white line c, which indicates the position of the 

 enamel, and will be seen to cover the upper surface 

 of the tooth dipping into the infundibulum, but not 

 to extend quite to the root. 



14. The uses of the ivory, enamel, and crusta petrosa 

 must be now noticed. The ivory is less dense than 

 the enamel, and harder than the crusta petrosa. On 

 the external surface of the incisor teeth, the crusta 

 petrosa is, by the attrition to which this part must be 

 subjected during the gathering of the food, soon 



