148 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



these lines form paths of least resistance, a fact taken 

 advantage of in operative dentistry. 



According to Noyes ("Dental Histology"), "the 

 enamel rods, or prisms, are long, slender prismatic 

 rods or fibers, five- or six-sided, pointed at both ends, 

 and alternately expanded and constricted through- 

 out their length. They are from 3.4 to 4.5 microns 

 in diameter, some of them apparently reaching the 

 entire distance from the surface of the dentin to the 

 surface of the enamel, but, as the diameter of the 

 rods is the same at their outer and inner ends, and 



as the crown surface is 

 much greater than the 

 surface of dentin cov- 

 ered by enamel, there 

 are many rods that do 

 not extend through 

 the entire thickness. 

 , These short rods end 



Fig. 105. a, Enamel rods in cross 

 section; b, Enamel rods, longitudinal in tapering points be- 



tween the converging 



rods, which extend the entire distance. To ex- 

 press this in terms of development, as the forma- 

 tion of enamel begins at the surface of the dentin, 

 the increasing area of crown surface requires more 

 ameloblasts, and as new ameloblasts take their 

 places in the layer the formation of new enamel rods 

 begins between the rods which were previously 

 forming. These short rods are most numerous over 

 the marginal ridges and the points of the cusps." 



The rods are not perfectly smooth and even, but 

 show alternately expansions and constrictions. 



