150 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



of growth, each dark stripe or expansion in a rod 

 representing a globule of calcified material. The 

 ameloblasts build up the rods by the addition of 

 globule after globule, surrounding them with a 

 cementing substance and completing the calcifi- 

 cation of both. In this sense the striation of the 

 enamel may be said to record the growth of the in- 

 dividual rods." 



Direction of the En- 

 amel Rods. Upon the 

 axial surface the rods 

 are usually straight 

 and parallel with each 

 other, but upon the oc- 

 clusal surface they 

 change their direction 

 by a series of sym- 

 metrical curves, and 

 often become much 

 twisted and wound 

 around each other, par- 

 ticularly the inner 

 ends. In operative 

 dentistry it is found 

 that the enamel upon the axial surface cleaves 

 readily while the gnarled portion upon the occlusal 

 surface tends to break away in chunks. 



Fig. no shows the general plan of the arrange- 

 ment of the enamel rods in the formation of the 

 crown. In the gingival half of the middle third the 

 rods are horizontal. Their inclination, from this 

 horizontal direction, gradually increases toward the 



Fig. 107. Tip of an incisor, 

 showing lines of stratification of the 

 enamel (Noyes). 



