DIGESTIVE SYSTEM. 151 



root and toward the occlusal surface, so that at the 

 apices of the latter surface the rods are placed 

 vertical to a horizontal plane. Because of this plan 

 Noyes divides all cavities, in dental work, into two 

 classes : 



"i. Those in which the enamel rods are in- 



Enamel. 



Enamel. 



Den/in. 



Dentin. 



Fig. 108. Straight enamel, Fig. 109. Gnarled ^enamel. 



showing dento-enamel junction showing dento-enamel junction 

 (Noyes). (Noyes). 



clined toward the cavity, characteristic of the oc- 

 clusal surfaces. 



" 2. Those in which the enamel rods are inclined 

 away from the cavity, characteristic of axial sur- 

 faces" (Fig. in). 



In preparing cavities on the axial surfaces the 



